Ethological and Autecological Studies on Canarian Vermileoninae (Diptera Brachycera, Rhagionidae = Leptidae)

Home , Brachycera, Canary Islands, Gran Canaria, Guanches, Maspalomas, Vermileo

Vidensk. Meddr dansk naturh. Foren. (1972) 135: 37-59

ETHOLOGICAL AND AUTECOLOGICAL STUDIES ON CANARIAN VERMILEONINAE (DIPTERA BRACHYCERA, RHAGIONIDAE = LEPTIDAE)

By J0RGEN FREDERIKSEN and AXEL M. HEMMINGSEN Strradam Biological Laboratory, 3400 Hillerrad, Denmark

CONTENTS

1. Introduction ...... 37 3. Adult ñies o f Lampromyia fortunata 2. Larvae of Lamprotnyia canariensis Mac- Stuckenberg, 1971 ...... 52 quart, 1838, aitd Lampromyia fortutrata A. Emergence ...... 52 Stuckenberg, 1971 ...... 38 B. Oviposition ...... 53 A. Distribution of habitats...... 38 C. The question of specific preferred B. Distribution of larval pits in Cenó- temperatures ...... 56 bio de Valerón ...... 39 4. Additions and corrections to Hem- a. The influence of temperature . , 42 mingsen & Nielsen (1971)...... 57 b. The influence of humidity.. .. 49 5. Summary ...... 57 c. The influence of light ...... 49 6. Ac kno wled gmen ts ...... 58 d. Correlations ...... 50 7. Re ferences ...... 59 e. Distribution of larval pits within caves...... 51

1. INTRODUCTION Previous studies in this laboratory have analysed 1) the feeding instincts of the larvae, the so-called worm-lions, which construct pits in dust similar to those of ant-lions (Hemmingsen 1963, 1968) and 2) the feeding, mating and especially the ovipository instincts of the adult flies of a variety of species of Vermileoninae from Italy, Spain, Tunesia and the 7 major Canary Islands (Hemmingsen & Regner Nielsen 1971; hereafter abbr. H. & N.). We refer to these papers for references to earlier authors. This paper is based mainly on studies in the free on the two Canarian species, Lampromyia canariensis Macquart, 1838, on Tenerife and especially L.fortunata Stuckenberg, 1971, on Gran Canaria in April-May 1971. The object was to study the influence of various ecological factors on the distribution of the larvae and on the oviposition of the adult flies, and also to compare oviposition in the field with previous studies by H. & N. in the laboratory.

3 35 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN The habitats of both imniature stages and ovipositing females are predominantly dust in niches, crevices or open caves of many sizes. But the fact that larvae in their pits are also found in dust at the base of walls and foot of palms, indicates that the primary requireinents of the species are fulfilled wherever there is sufficient shelter against the wind for dust to collect; while shelter from sun and'rain appears to constitute secondary requirements. For the adult flies the presence of flowers in the neighbourhood from which to suck nectar, is also important.

2. LARVAE OF LAMPROMYIA CANARIENSIS MACQUART, 1838, AND LAMPROMYIA FORTUNATA STUCKENBERG, 1971 A. Distribution of habitats We found caves inhabited by worm-lions from near sea leve1 to about 930 m altitude (above Chio and above Aguamansa on Tenerife). Fig. 1 shows that there is no partiality for the opening of the caves inhabited by worm-lions to face any special points of the compass. Also more than 100 larval pits at the foot of Canarian palms (Plioenix canariensis Hort.) in the palm grove in El Charco at Maspalomas, Gran Canaria, were found to be situated independently of the points of the compass. It may not be amiss to state expressly that in al1 localities dealt with in this paper we have established with certainty that at least some of the pits found con- tained worm-lions. The worm-lion pits cannot at a mere glance in al1 cases be distinguished from ant-lion pits, though generally the former have steeper walls and occur in finer dust. Where the sun occasionally penetrates into the caves larvae may sometimes be found, yet as a rule exclusively in the shade. At the foot of palms larvae were found up to 1.5 m from the palm, yet always in its shade. Where the caves are facing North the substrate may at times be more or less humid causing the larvae to become more limp and sluggish at least at times. Substrates from caves inhabited by worm-lions on Tenerife (3) and Gran Canaria (5) were most obligingly examined by Mr. Henner Bahnson, Geological Survey of Denmark. Apparently largely resulting from rock disintegration these substrates consisted of poorly sorted sand, with no regular differences between the weight frequencies of grain sizes below 1/16, 1/16-1/8, 1/8-1/4, 1/4-1/2, 1/2-1, 1-2 and above 2 mm, some even apparently tending to be equally distributed over these groups. Still, some of the samples afford evidence of sorting by wind, the weights of grain sizes, which range from below 1/16 to above 2 mm, being log normally distributed with the peak of sizes between 1/4 and 1/2 mm. STUDIES ON CANARIAN VERMILEONINAE 39

Fig. 1. Orientation of worrn- N lion-inhabited caves in Tenerife and Gran Canaria. Each small square represents a cave or a cave complex facing the respective point of the compass.

GRAN CANARIA S

We have collected larvae in a number of localities other than those listed by H. & N. (legend of fig. 1 on p. 151). Some of these are situated so near some of those already marked in the figure quoted (Nos. 17, 18, 24, 27 and 28), that they would in the map practically coincide with them. Others fill in the gaps left, viz. on Tenerife above San Juan de la Rambla and west of Garachico on the North coast, and Güimar on the South East coast, and on Gran Canaria Arguine- guin and between Playa de Mogán and Mogán in the South West part. The flies emerged at the time of writing were, as expected, from Tenerife L.canariensis and from Gran Canaria L.fortunafa; including, however, from the “gaps” so far only two from Garachico and one from the road between Playa de Mogán and Mogán.

B. Distribution of larval pits in Cenóbio de Valerón An ideal locality for studying both larval and imagina1 biology of the Gran Canaria species Lainpromyia for.tunata was the remarkable prehistoric assem- blage of caves called Cenóbio de Valerón (Fig. 2) facing S.E. about 280 m above sea leve1 near Guia in Northwestern Gran Canaria; ideal but for the frequent visits of tourists. The aborigines, the guanches, are said to have elaborated these caves on the base of cavities already preformed by nature and to have used them for various piirposes; according to one version, as granaries; according to another version, as abodes for virgins to be fattened prior to marriage. Cenóbio

3* Number of Average number Distance Number of larvae in caves Number of larvae per of larvae per from of 4 different sizes larvae Storey storey (corr. cave (corr. to flowers per storey 1 1 11 1 111 1 IV to area of IV) area of IV) m

* * O O O 28 3 2-0 O 5 8.7 2.2 26 1-8- 9 12.0 3.0 22 0-0 40- 17- 13 92 175.3 35.1 20 13-9 26 46-1 I- 169 190.8 34.7 18 33-19- 9-25 12 >lo0 >112 >116 > 58 15 17-19- 24-0t 60 96.0 32.0 10 ot-ot 10 >50 16 > 76 > 156 > 52 5 38-50 88 352.0 172.0 2

* Several large caves without pits. Disinfected cave.

means cloister. Actually, the caves are like small rooms or cells rather than natural caves, Some are so low that one can only couch in them, others are so dark as to be fit only to sleep in. Many walls and roofs of the original 583 caves have collapsed and left 365 caves more or less intact (information by the guardian of the caves). Of some caves only the bottom is left. The present caves (when walls and ceiling are intact) measure from 3 (width) x 1 (length) x 1 (height) m to 3 x 33 x 2%m. One cave of intermediary size measured 13 x 13 x 14 m. Most caves are not visible in Fig. 2, being situated on both sides of a sort of high “corridor” extending deep into the dark rear to the right in the picture. Formerly, when there was no guardian, these more or less hidden, dark caves (as also a few in the front, see Table 1) were often polluted by visitors, but had now been disinfected, which is why they house no worm-lions. No ant-lion pits were found in Cenóbio de Valerón, and the worm-lions were usually directly visible at the bottom of the pits, as described and pictured e.g. by Hemmingsen (1963). The autecological studies of the larvae reported in the following pages were substantially planned and carried out by the junior author (J.F.). STUDIES ON CANARIAN VERMILEONINAE 41

Fig. 2. Cenobio de Valerón, Northwestern Gran Canaria. In the huge cave (a) the snialler caves or cells (examples in b) fornierly used by the aborigines and Hhich house nunierous larval pits of Ln/iipron-.iafortlr,rnro, are seen. There are a larger number further back to the right in the huge cave. Average size of the caves about if x if x l+ ni. A.H. phot. 42 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

Table 1 shows the distribution of larva1 pits according to a rough estimate of proportional cave sizes (1-IV) and storeys (1-9). With rising storey number the caves in the front part of the whole complex are in general - though not very regularly - situated more and more to the rear, more or less like the arrange- ment of seats in an ainphitlieater. It will be seen from Table 1 that there is a relatively large number of pits in the louer storeys, especially in the lowest one, with a decrease throughout the storeys. There is a steep fa11 in numbers between the 6th and 7th storey, and already in the 9th storey there are no more pits. The density of pits, e.g. as measured in terms of number of pits per 100 cm2 in the places where there was niaximal density, was decidedly greater in the 1st and 2nd storey (two caves in each storey had 9 and 13, and 10 and 13, larvae per 1CO cm2, respectively) than in the other storeys, where there was lesc than 1 pit per 100 cm? (yet in one cave in the 4th storey 5 in the middle and 9 at the wall per 100 cm?). This is taken to indicate more frequent ovipositions in the lower storeys. It is an obvious question to ask to what extent the distribution of pits may be correlated with such ecological factors as temperature, humidity and light. Measurements and samples to estimate these factors were collected in caves along an imaginary line throughout the storeys on a day with a clear sky with sun, 19th May, 1971, 11.30-12.00 a.m. The line skipped caves in lst, 3rd, 6th and 7th storeys. The figures obtained are taken to hold with some approximation relatively to one another, with sonie change throughout the season of oviposition. An idea of the rise in temperature May-July in the surface of the substrate in one of the caves in the 2nd storey can be obtained from Table 2. a. The influence of temperature The temperature (Table 3) was measured by Hg thermometers corrected with reference to a standard therniometer. 1n the caves into which the sun-rays did not penetrate, the measurements were made on the substrate surface 1) in the middle of the floor, 2) at the wall and 3) in the air. In the caves into which the sun-rays penetrated, measurements were made in the sun-exposed area 1) on the surface (O cm) and 2) 1 cm below the surface (-1 cm); on the surface in the 3) shade at the shade border, 4) in the middle of the shaded area and 5) at the wall; and 6) in the air. In the shade there is never any difference between O cni and -1 cm. Jn the cave coniplex as a whole the sunlight did not reach storeys above the 6th on account of the overhanging rock. As seen froin Table 3 the effect of this is that only within each sun-exposed cave (storeys 2-5) there is a marked tenipera- ture gradient. The difference between the tliree caves in the three lowest storeys - runniiig couiiter to what oiie miglit perhaps expect - is largely due to different periods of exposure to the sun but also to different widths of the cave openings. STUDIES ON CANARIAN VERMILEONINAE 43

Table 2. Maximum and minimum temperatures in the shade of the surface of the substrate in cave in 2nd storey.

Temperature "C Date 1 Min. 1 Max.

18.V 16.5 21.5 21.v 16 21 24. V 16.5 21 29. V 16 22 5. VI 16.5 24 12. VI 17 23 19. VI 16.5 22.5 26. VI 17.5 22.5 3.VII 17.5 22 1O.VII 18 23 17.VII 16 24 24. VI1 19 24

Table 3. The variation of cave temperatures along a line throughout the different storeys of Cenóbio de Valerón.

Temperature "C

Substrate Storey -- Comments In sun -- Inshade Ocm Ocm 1 - 1 cm 1 Border 1 Middle 1 Wall

18.0 18.0 18.0 No sun; no larvae 19.5 19.5 21.0 No sun 45.5 50.0 35.0 22.5 22.5 This cave had longest period of exposure to sun. 42.5 40.0 30.0 21.0 21.0 22.0 39.0 38.5 23.5 20.5 21.5

The absence of sun exposure in the upper storeys brings about a nearly uniform temperature throughout the cave. The directly sun-exposed hot areas (storeys 2-5) are doubtless unfavourable to oviposition, which in nature always takes place in the shade (though in the laboratory ovipositions were attained at up to 35°C in the shade and at 26°C in the sun when there was no shade (H. & N.)). On overcast days ovipositions were observed in areas which on sunny days were sun-exposed (45-50" C). The eggs laid there presumably perish from desiccation. Oviposition in nature was observed at temperatures of 18-21.5"C. A temperature of 18°C was observed in the 9th storey (Table 3) and is likely to be- 44 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

Te m p. OC Fig. 3. The variation of tem- LT Y perature across the border be- 51 a O tween sun-exposed area and SHAOE *-m SUN shade in a cave in the 4th storey n r of caves in Cenóbio de Valerón.

22.8'

20 I PITSi 4 L I I l I l O 50 100 cm

Distance from beginning temp. giadient

come even a little higher later in the season, so the absence of pits there cannot be explained from a too low temperature for oviposition. The distribution of the larvae in the cave complex must necessarily depend not only on the site of oviposition but also on the subsequent sites taken up by the larvae after hatching, e. g. after wandering about or perhaps leaping. The temperature preferred by the larvae has been studied 1) in the special cave in the 4th storey represented in Table 3, 2) in the laboratory both on Gran Canaria on freshly collected larvae and in Denmark in the Zoological Labora- tory, Copenhagen University, on larvae brought to Denmark from Cenóbio de Valerón, fed on ants ti11 the onset of winter. then starved at about 20°C until the experiments had been concluded 17.IL-l.IV. 1971. 1) The respective cave was 1.5 m high, 1.5 m broad and 3.5 m long (deep). The floor was covered by a fine homogeneous layer of dust. Near the opening of the cave the border between sunlight and shade formed an arc from one side of the cave to the other, convex toward the interior of the cave. In the shade along this border line more than 100 larvae had congregated and built pits in a belt. The arc was practically constant throughout the day. But mornings and eve- nings pits at the extreme ends of the arc were exposed to the sun though only for a short period (42°C for 2 hours). We think that when pits are observed in STUDIES ON CANARIAN VERMILEONINAE 45

Fig. 4. Comparisons between the rnea- sured maximum temperatures in the - 26 - surface of a cave in the 2nd storey of - - Cenobio de Valerón at 280 m altitude 25 (thick he); and monthly averages of - - maximum temperatures at 547 m alti- - - tude at La Laguna, Tenerife (from Ce- - - ballos & Ortuño). - -

20 ------

16 1111111111

sun-exposed areas, e. g. on Fuerteventura, and elsewhere occasionally near openings of caves, this can only be sustained by the larvae for short periods. In the rear dark part of the cave single pits were scattered over the floor; and some in a row along the wall, interrupted here and there. Fig. 3. shows the variation of temperature across the shade border, based on the three measurements in storey 4 given in Table 3, and the position of pits in this gradient. The bulk of pits were situated nearest to the border. The gradient begins 0.5 m behind the shade border as shown in the figure. In the rest of the cave the temperature was 21°C at the surface. At this early point of time of the season the larvae can attain the preferred temperatures only in the shade border area. But throughout the summer these are normal temperatures. This is evident from Fig. 4, which shows the mean maximum temperatures at a somewhat higher altitude on Tenerife (from Ceballos & Ortuño 1951) and the values from Cenóbio de Valerón (O cm in the shade - 2nd storey) measured mainly after our departure by the guardian of the caves, Señor Adolfo Delgado Reyes (Table 2). 2) The device employed in Denmark to determine the preferred temperature measured 100 x 10 cm. The substrate had particle sizes of 0.4-0.5 mm in a layer 2 cm deep. The temperature in the sand was measured by means of thermistors. In each test 5-1 1, mostly 10, large larvae, presumably al1 in the last instar, were placed on the substrate where they soon disappeared in the manner previously described (Hemmingsen 1963, pp. 241 -242). Tf the larvae are placed in the apparatus without a gradient, in the course of some hours they will have scattered immediately about the place; and in the course of some days, built pits al1 over the area. 46 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

To leave them time to adjust themselves to the constant gradient the position of the pits was not recorded until after 2 days. After 2 days the larvae had wan- dered and finally come to rest and built pits in the preferred position. Once settled there they stuck. As it turned out that the larvae through wanderings avoided temperatures above 40°C they were iii part of the tests placed in the gradient at so high temperatures (36-42" C) that they were induced to wander before pit construction. In other tests they were placed at 26"C, that is within the range of temperatures found in the 4th storey, without any significant difference in the results (Table 4). Fig. 5a shows the resulting distribution in a single test (No. 5 in Table 4). As will be seen there was in this test some variation in temperature difference between thermistors placed at equal intervals. With the large temperature gradient (10-50°C) the range of preferred temperatures becomes in Fig. 5a rather narrow ; and, therefore, the calculation by interpolation between the thermistors of the position of the larvae at the single temperatures apparently leads to a greater scatter in Table 4 than Fig. 5a perhaps suggests. The narrow range of preferred temperatures also entails that the larvae disturb one another at the pit construction, and this may influence the scatter about the preferred temperature. How- ever, Table 4 and Fig. 5b show that nevertheless the larvae have predominantly (80 %) grouped themselves within the same range of preferred temperatures as in tlie cave in the 4th storey (Fig. 3), thus providing further evidence of the depen- dence on ternperature of the distribution of the pits. The temperature below which 50 % of the preferred temperatures fell, was 22.8"C (Fig. Sb). In both cases more pits were built near the upper temperature limit. It is well known that many other larvae have a preferred temperature which accords closely with that of their normal environment. This also often means the temperature of their normal breeding place. But the preferred temperature may be greatly influenced by previous conditions such as thermal history, season, humidity and nutritional state (cf. Wigglesworth 1953, pp. 141-143). As regards the preferred temperatures in point the previous environmental temperature may have been low in the cold season, but had presumably been for weeks before our visit about the same as in the laboratory (about 20°C in the shade). The tests were made in the free on Gran Canaria on May (day length about 13+ hours, sun's culmination height about 80"; from Hemmingsen 1951, fig. 38 on p. 103) and in tlie laboratory in Denmark February-March (day lengths 93- 13 hours, sun's culniination height about 30"). lnasmuch as day length may be a denominator of season there has not been much difference, at least as far as the last laboratory tests in March are concerned. The humidity has in the prece- ding months or weeks doubtless been low both in the dry dust in the free (cf. Table 5) and the dry sand in the laboratory. In the free we do not know the nutritional state. We did not forin the impression that food was abundant in Table 4. Thermopreferendum tests with larvae of Lnmprumyiu furtirnatn. (1 7.11-1. IV. 1971)

v) Number of larval pits at the different temperatures after 2 days 4 Fll Remarks C tests ** 10 11 12, 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 U \ m3 v) O z n P Z P 76 i-i P z c rn

~~ ~~ ~ ~ ~~~ id

Suni ...... 2. . 32. 1 .241879941524. . I 3 r Percentagc m O of total .... 2.7 ..4.1 2.7 . 1.4 . 2.7 5.4 1.4 10.8 9.4 12.2 12.2 5.420.3 2.7 5.4 ..1.4 z 3 Z * Tcmperatiire in apparatus. P ** Temperature where larvae were placed in the apparatus at the beginning of each test. m 48 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

\ a Fig. 5a-b. Distribution of i O OCO :-pit Ihermistor O 0 -Sand preferred temperatures in

Cumulated Number 11 percentage larvae

100 20

80 15 70

50 10

b 40

30 5 20

10 2 1 - 10 15 20 fi 25 30 35 2 2,8 Temp.OC

the caves; but though the larvae can survive for at ieast 3 years when food is scarce the abundance of pits suggests that food must be available at least at times. Since there was thus some differences in previous conditions but these differences appear not to have been great, it is difficult to decide whether the accordance in preferred temperatures in the free and in the laboratory are due to the preferred temperatures not being much influenced by previous conditions or to the differences being comparatively slight. STUDIES ON CANARIAN VERMILEONINAE 49 In both cases, however, the preferred temperatures found appear to correspond to the natural conditions in spring. As to whether there are seasonal varia- tions in preferred temperatures, we know nothing. The steep fa11 in number of larvae from 6th to 7th storey (Table 1) coincides with the limit for penetration of direct sunlight. At no point of time this penetrated to the 7th storey. It thus might seem that only in the storeys where the caves are exposed to temperature gradients like those in storeys 2, 4 and 5 (Table 2) do we find numbers of larvae as in storeys 1-6. The temperature found in the pit-free 9th storey (Table 3: 18’C) is at the lower limit of the temperatures preferred by the bulk (80 %) of larvae (Fig. 5). Later in the season the temperature may be somewhat higher, but in the colder seasons still lower. And though, at certain low temperatures many larvae in the laboratory survive more than one winter, it seems posible that the number of days with effective temperatures may not be sufficient for normal ontogenesis there. Even into caves elsewhere facing North (Fig. 1) the sun-rays may sometimes penetrate at least to front parts of the caves. b. The influence of humidity The humidity of the substrate was measured in terms of weight percentage water content. The relative humidity of the air was measured by a Lambrecht hair hygrometer adjusted before each measurement. Table 5 shows that in the sun- exposed storeys the substrate humidity varies in accordance with the temperature variation (Table 3). But it is remarkable that the upper storeys have so low values that they correspond to the values for the shade border of the lower storeys which have considerably higher temperatures. This may be ascribed to differences in flow of humidity from the subsoil, the upper caves being placed honey- comb-like above one another whereas the lower ones are directly embedded in the rock. Thus there is not, as one might have expected, a rise in substrate humidity, toward the rear upper storeys. As, furthermore, larvae were found down to a water content of 2.5 % (8th storey; no larvae in shade middle with 1.8 %), no correlation of the distribution with humidity can be seen. c. The influence of light Light (Table 6) was measured with a lux meter (“Gossen”), the photo cell being placed horizontally on the surface of the cave floor. Here again each cave in the lower storeys shows a marked gradient. The small differences between the three lower storeys must be due to different widths of the cave openings. 50 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

Wcight percentage water content Percentage relative Storey humidity of air Insun 1 Shade border 1 Shademiddle 1 At wall

9 3.9 4.3 8 1.8 2.5 60 5 1.4 1.9 4 2.6 3.9 6.5 50 2 2.8 4.9 6.5

Table 6. The variation in light in caves along a line throughout the different storeys of Cenobio de Valerón.

Luminosiry íin 1000 lux) Storey In sun 1 Shade border 1 Shade middle 1 At wall

0.35 0.35 1.40 1.40 52.0 1.60 0.50 48.0 2.65 1.30 44.0 - 2.60

There was no light orientation of the position of the larvae in the pits. They lay in al1 directions. Oviposition has been observed in a cave at 150 lux, so the absence of pits in the 9th storey where 350 lux was measured, cannot be due to too low illumination for ovipositions. This, of course, does not rule out the possibility of a correlation between the tendency to lower illumination (Table 6, col. 5) and decrease in number of larvae toward the rear caves (Table 1). The frequency of ovipositions might decrease with decreasing luminosity. d. Correlations It thus seems that, at least in the present cave complex, a temperature gradient conditioned by the presence of sunlight is important for the larvae, excluding or reducing number of pits in the upper storeys; and that the lower the illumination the fewer larvae. The causal relationships are, however, difficult to evaluate. As regards particle sizes no correlation could be established. The possibilities of prey falling into the pits may play an unknown role for the distribution. Experiments with artificial pits in the form of thimbles gave no clues, no "preys" falling into them. A few collemboles and tenebrionides were seen in the caves. A distinct correlation is found, however, between number of larvae or density STUDIES ON CANARIAN VERMILEONINAE 51 of pits and distance of the caves from the flowers in front of the cave complex (Table 1, last col.), where the flies feed and presumably mate. The larger numbers of larvae and the greater density in suited substrates in the lower storeys suggest that the frequency of ovipositions decreases with the distance from the flowers. e. Distribution of larval pits within the caves After hatching, the larvae disperse by wanderings or leaps. There is, however, not much chance for them to leave the caves because as a rule the floor is lower than the surroundings. As mentioned, there were in the cave dealt with above, in the rear part of the cave, a series of pits along the wall and some scattered on the floor. ln order to find out how the larvae in that part wander, two experiments were made, in both of which larvae were placed in a dust heap in the middle of the floor in the dark part of the cave. Next day they were found to have left it in al1 directions, also parallel to the walls. Some had then built pits, while others had con- tinued their wandering. There was no inkling of partiality for wandering toward the light (warmth) or toward the wall. If the larvae wander by night some must get into the area exposed to the sun by day. The heating by the sun will induce them to again presumably wander at random. Some will reach the shade. But unless they are chased towards the shade during an advancing sun-shade border for a shorter period in the morning, it seems that there is no gradient in the sun-exposed area to prevent some of them from wandering away from the shade and perish. This point needs further study. The apparent preponderance of pits at the walls of caves or containers, mentioned by earlier observen, arises because sooner or later the larvae reach the wall where their possibilities of continuing their wandering become reduced by 180". They will, therefore, more frequently move along the wall than away from it. This phenomenon depends on the size of the area and the number of indivi- duals. lf the area is reduced, i.e. the density of pits increased, a greater number of larvae will get into the middle again; and byfurther reduction of area a stage of uniform distribution is reached. Nor do ovipositions take place with partiality for the wall. The density of larval pits must depend on particle sizes. ln very fine dust the pits can be more closely adjacent than if the dust contains grave1 or pebbles. In a dense population the larvae may disturb one another in throwing off mate- rial (cf. Youthed & Moran 1969, for ant-lions). The dense populations we have observed were precisely in localities with powdery homogeneous dust. Examples : At Arguineguin. in the South of Gran Canaria more than 50 pits on ca. ;5 m? in a low cave facing S. E. in a barranco (ravine); on the road from the South coast to Mogán we found several very small cavities with 5-6 larvae in each; in Cenó- 52 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN bio de Valerón, 1st storey and 4th storey, where there was a dense population in the front part of the cave and in the rear part typical transition to caves with only a few larvae along the wall; in the 5th storey a cave shaped as a tub (the ceiling and most of walls had disappeared), where the floor was covered with a substrate of much varying particle sizes, 25 pits were distributed over the 2-3 m2 large floor. Though this “cave” was situated in the entrance to the upper caves no tourists had stepped into it. The reason why so few caves had pits in the middle must be sought in the visits by tourists. Only larvae placed along the walls had a chance to survive in such overrun caves. Against partiality for walls speaks also the presence of several hundred larvae in a 1 m2 large dust heap in the shade under palms about 1.5 m from the trunk in the palm grove El Charco, at Maspalomas at the South end of Gran Canaria. Conclusion: After hatching from the egg or when at any other time they are induced to move, the larvae wander about in the cave at random. If during the wandering they get exposed to a temperature gradient they stop and build pits at about 23”C. This temperature approaches the conditions prevailing most of the summer. Notwithstanding the agreement between the temperature in the shade near the shade border and the preferred temperatures in the thermopreferendum apparatus it might be argued that it may not be the preferred temperatures that stop the larvae at the border but the sunlit hot area beyond it. It seems natural to suppose that both factors are operating, being two aspects of the same phenomenon, neither of them excluding the other one. It should be emphasized that we feel that our results should be checked under other conditions, e.g. at other seasons, both colder and warmer, and on Lampro- myia hemmirigseni Stuckenberg, 1971, on Fuerteventura, where pits are found along unsheltered walls.

3. ADULT FLIES OF LAMPROMYIA FORTUNATA STUCKENBERG, 1971 A. Ernergence Judging from observations both in the free in the Canary Islands and in the laboratory at Strodam, Denmark, pupation starts in April. Numerous larvae collected on Gran Canaria, brought to Denmark, there fed until the beginning of winter, and then starved at about 20°C until April, then began to pupate without food having been offered. From winter solstice until 1. May, day length increases at the latitude of Gran Canaria from 109 to 139 hours; and at the latitude of Stro- dam laboratory, Denmark from 7 to 15.5 hours (from Hemmingsen 1951, fig. 38 on p. 103); and it seems likely that it is increasing day length that brings about pupation. In accordance with this supposition larvae of a different species brought STUDIES ON CANARIAN VERMILEONINAE 53 from the southern hemisphere, where the flight period is October-January, emerged in Denmark May-August (H.& N., p. 156). In April 1971 up to 20. April when we left Tenerife no adult flies, let alone ovipositing females, were observed there; in half the caves no doubt because the temperatures measured in the caves where larval pits were found was only 9.5- 17.5"C. But .on Gran Canaria adult flies were observed from 3. May 1971 (when we arrived) in niches in a wall in Barranco del Guiniguada, where also a mating was seen. B. Oviposition In the caves of Cenóbio de Valerón one or more ovipositing females could usually be studied every day, whereas at other minor localities much time would be wasted in waiting for a sufficient number of females to appear and oviposit. A summary of the most important results on oviposition have already been published (H. & N., postscript on p. 202). Flies on the wing were seen outside the caves from 17°C on overcast days. The erratic horizontal flight within a small space characteristic of the swarming of many insects was seen among plants in one or a few males at 28°C in the free (as also under similar conditions in the containers in the laboratory). We saw no females ovipositing before 11 o'clock; nor after 16 o'clock. The air temperature when ovipositions occurred was in the respective caves 18-21.5"C. Substrate surface temperature was always 0.5-2.0" C, average 1.25" C, higher. One cm below the surface the temperature was from 0.5" below to 1°C above surface temperature, average 0.21 O C. The measured illuminations at the sites and times of oviposition were 150- 9000, usually 1600-3300 lux, i.e. usually in the same region as in the laboratory experiments (H. & N., table 1 on p. 155). The same applies to the relative humidities (50-70 %). In the caves of Cenóbio dc Valerón the ovipositing females might face any points of the conipass (Fig. 6), i.e. with no partiality for the lighter entrance of the cave or the darker background; and the same was the case with flies resting between ovipositions in other sites (Fig. 7). The females might oviposit at any sites in a cave including sites close to the wall. The ovipository behaviour as observed through binoculars at 1-2 m distance was essentially as described in the laboratory experiments by H. & N. During a digging phase 1 the middle pair of legs, accompanied by forwardly and down- wardly directed thrusts in the air of the abdomen, dug a pit in the dust by back- ward movements. At the end of phase 1 the egg was laid in the pit;' and during phase 2, the pit was covered by movements of fore and middle legs. We could see no jerks of fore and middle legs at the end of phase 2, as seen at higher temperatures in the laboratory. This agrees with the observations in the laboratory at about 20-22" C.

4 54 JDRGEN FREDERIKSEN 19 AXEL M. HEMMINGSEN

NW Fig. 6. Orientation of females during oviposition in the caves of Cenóbio de Valerón. Each srnall square represents an oviposition during which the female faced the respective point of the compass. The caves in question, like the whole huge cave, faced S. E.

NW I Fig. 7. Orientation of females 1 during interovipository rest in the caves of Cenóbio de Valerón. Each small square represents an observation of a female facing the respective point of the cornpass. In a few cases, not included, the BB N E fernale rested on the wall of the cave. The caves in question faced S.E.

SE The durations of phase 1 are compared in Fig. 8 with the durations at 20-22" C in the laboratory. There is a remarkable agreement. Also the rates of abdominal thrusts compared well with the laboratory data (Fig. 9), coinciding with them within the largest part of their range, though they may perhaps tend to be higher. In both cases the rates have been calculated as STUDIES ON CANARIAN VERMILEONINAE 55

Length 01 phire 1. smin. Fig. 8. Comparicon between the lengths of c phase 1 (ovipository pit-digging) of Lnm- L . . 3 200 t- proniyin fortirtiatn in the free and in the .o laboratory. The indoor data were obtained . on flies reared in Denmark from larvae frorn Gran Canaria (H. Br N., fig. 16 on p. 183). A. Cenóbio de Valerón, 18-21°C. 0.. B. Laboratory, ordinary daylight in room without sun. C. Laboratory, 500 watt incandescent lamp, 20.5-21.5" C. . ;+ 100 . .I t .o e t ** 0. 0. 70 t :. .

0.i 0:. e. 50 loe 0... . 40 ...0..

30 1; l A B C number of thrusts divided by the total length of phase 1, though the thrusts may fairly often have ceased sooner or later toward the close of the phase. Calculated only for the part of the phase in which the thrusts were exhibited the rates would of course be higher. Still for the but 9 ovipositions in the free and the but 5 in the laboratory in which the fraction of the phase with thrusts was directly established, the higher rates in the free, 0.35-0.56, fa11 within the corresponding range of points in Fig. 9, whereas out of the higher rates, 0.37-0.72, of the 5 in the laboratory 3 fa11 above 0.60. Calculated from the entire period the 9 rates in the free are 0.28-0.38; the 5 rates in the laboratory, 0.10-0.47. Evidently, this does not, make any difference as far as the present comparison is concerned. The length of phase 2 varies in 22 ovipositions from being immeasurably short to 9 cmin. (= centiminutes - hundredths of a rninute), generally 3.5-6 cmin., average 4.8 cmin. This again agrees well with the observations in the laboratory at 20-22°C: 1.5-11, average 5 cmin. (H. & N., p. 174). In these respects oviposition in the natural, partly dusty substrates (from below 1/16 to 2 mm) thus appears to be much the same as in the sifted sand (0.4- 0.5 mm) as used in the laboratory.

41 56 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

Fig. 9. Comparison between rates of forward and downward abdominal thrusts in the air of the ab- . dominal apex during ovipository pit-digging (phase 1) of Lnnipronyin fortiiiintn in the free and in the la- . . boratory. The indoor data were obtained on flies . . reared in Denmark from larvae from Gran Canaria . 0. . . (H.& N., fig. 17 on p. 185). A. Cenóbio de Valerón, 0. 18-21.5"C. B. Laboratory, 20-21 "C,ordinary daylight H. 0. or 500 watt incandescent lamp; cf. this paper, chapter .u.. y 0. . . . 4, on p. 57. 0.. .

A B

In the laboratory this species - in contrast to L. canariensis - did not start ovipository digging (phase 1) immediately after settling from interovipository flight but waited for a short while. This was not so consistently the case in the caves, presumably because the substrate was not so homogeneous as the sifted sand in the terraria. Presumably owing to the inhomogeneity of the substrates, from fine dust to small pebbles or even hard rock, females usually apparently tested in flight the substrate with the hind legs in severa1 places before they found a sufficiently fine dust for oviposition. And then she started at once. The same was actually seen in the experiments with a coarser substrate recorded in the laboratory (H. & N., p. 188). Another trait was that there were numerous exceptions to the staying on for a while after phase 2, which in the laboratory is seen in this species in contrast to L. canariensis. The latter species ñies away immediately. The pause between ovipositions varied between 1 and 17 minutes. Samples of the substrate at the sites where females tested the substrate but left without ovipositing, and at the sites of actual ovipositions, were collected and the particle sizes measured by sifting methods in The Geological Survey of Denmark (Danmarks Geologiske Unders~agelser).No differences could, however, be established. Presumably the samples were not small enough to represent exactly the very limited spots tested by the tips of the hind legs of the fly.

C. The question of specific preferred temperature As seen in fig. 13 on p. 173 of H. & N. the specific length of phase 1 decreases apparently in the following order of sequence of species : L.pallida, L. hemmingseni, L. fortunata, L.funebris, L. canariensis, Vermileo nigriventris. Except for L. hem- STUDIES ON CANARIAN VERMILEONINAE 57 mingseni and L.funebris the sequence is well established. It might be asked whether this were something to do with the temperatures at which the respective species oviposit, the sequence being that of apparently decreasing thermophily. L. canariensis is, however, unlikely to oviposit at temperatures lower than those at which L.fortunata was observed to oviposit in the free, since the Cana- rian species are al1 largely inactive at temperatures below those in point (1 8-21.5"C). No doubt later in the season L.furtunata oviposits also at higher temperatures. The measurements undertaken in one of the caves in Cenóbio de Valerón after our departure by our friend Señor Adolfo Delgado Reyes, guardian of the caves, and reproduced in Table 2, showed that maximum temperatures in June-July 1971 varied from 22-24°C; and at least L. canariensis is known to be on the wing ti11 June and July (cf. H. & N., p. 197).

4. ADDITIONS AND CORRECTIONS TO HEMMINGSEN & NIELSEN (1971) Fig. 9 on p. 165: By printer's mistake inverted; corrected in reprints. Fig. 17 on p. 185: 1) Only 5 points of 0.44-0.54 represent experiments with the 500 watt lamp; the others, experiments in ordinary daylight. 2) The inference that ultraviolet light and additional strong illumination enhances the rate of abdominal thrusts during ovipository pit-digging was corroborated in 1972 in direct observations, i.e. not via films: With the Philips HP 125 watt mercury lamp placed a short distance above the wire gauze ceiling of the container employed for oviposition studies (H. & N., fig. 2) in 7 ovipositions at 25.5-26.5"C0.83 -0.98;in one, 0.61 ; and with the 500 watt incandescent lamp added about -$ m above the gauze in 10 ovipositions at 26-28°C 0.87-1.24 thrusts per cmin.

5. SUMMARY Field observations on larval worm-lions showed no correlation between cave openings and points of the compass. The same applied to habitats at the foot of palms. Caves are common habitats, because disintegration products in the form of dust or fine sand, partly derived from the cave walls and ceiiing, and partly brought by the wind, collect and settle there. Other worm-lion localities, e.g. the foot of palms, exhibit similar sheltering properties. Studies in the cave complex Cenóbio de Valerón, Gran Canaria, showed a positive correlation between luminosity and number and density of larval pits; with especially few or no larvae in caves into which the sun never penetrated. 58 J0RGEN FREDERIKSEN & AXEL M. HEMMINGSEN

There was a negative correlation between number and density of pits and distance from flowers. The preferred temperatures of the larvae was the same in the caves, where they were in the shade at the border between sun and shade, and in the laboratory (18-30°C with 50 % of the preferred temperatures falling below 22.8"C). After hatching or when being placed in the middle of a cave the larvae wander about at random. The apparent preponderance of pits at the walls of caves or containers arises because sooner or later the larvae reach the wall where the possibilities of continuing become reduced by 180". They will, therefore, more frequently move along the wall than away from it. If during wandering they get exposed to a temperature gradient they stop and build pits at about 23" C. This approaches the conditions prevailing al1 summer. The females niay face any points of the compass during oviposition and interovipository rest, i. e. with no partiality for the direction of light. The Gran Canarian species, Lampromyia fortunata, was observed to oviposit only in dust and in the shade, at 150-9000, usually 1600-3300 lux, at 18-21.5"C and relative humidities of 50-70 %. As the Canarian species are largely inactive below 18" these observations im- ply that this species does not consistently oviposit at higher temperatures than L. canariensis. The length of phases 1 and 2 and number and rates of abdominal thrusts were found to practically coincide (phases) or at least to coincide for the largest part of their ranges (thrusts) with the observations recorded in the laboratory (H. & N.) at 20-22°C with sifted sand as substrate. Oviposition in the natural partly dusty substrates and in sifted sand thus appears to be much the same. There were, however, many exceptions to the short delay in start of digging after settling seen in the laboratory and to the staying on after phase 2.

6. ACKNOWLEDGMENTS We are indebted to the Carlsberg Foundation and the Nordisk Insulin Foun- dation for financia1 support, to Señores Don Jaime O'Shanahan and Don Rafael Cárdenes Suárez for providing laboratory premises and facilities in Las Palmas de Gran Canaria, to Dr. Ellinor Bro Larsen for loan of the thermopreferendum apparatus in the Zoological Laboratory of Copenhagen University ; to Dr. Helge Gry and Mr. Henner Bahnson, Geological Survey of Denmark (Danmarks Geo- logiske Underssgelser), for the measurements of grain sizes of substrate, and to Mrs. E. Nyhave Kristiansen for preparation of diagrams for reproduction. We also wish to thank Dr. B. R. Stuckenberg, Natal Museum, South Africa for raising in litt. some of the questions which we have tried to answer. c

STUDIES ON CANARIAN VERMILEONINAE 59

7. REFERENCES

Ceballos, L. & F. Ortuño, 1951 : Vegetación y flora forestal de las Canarias Occidentales. -Madrid. Hernrningsen, A. M., 1951 : Observations on birds in North Eastern China especially the rnigra- tion at Pei-tai-ho Beach. - Spolia zooi. Mus. haun. XI. 1963: The ant-lion-like sand trap of the larva of Lnmpromyia cntiariensis Macquart* (Dip- tera, Leptidae = Rhagionidae, Verrnileoninae). - Vidensk. Meddr dansk naturh. Foren. 125: 237-267. * Should now be corrected to Lnmproniyin fortunatu Stuckenberg, 1971. 1968: A review of instinctive behaviour in the worm-lions Vermileo vermileo L. and Latii- promyiu pullidrc Macquart (Diptera Brachycera Rhagionidae, Verrnileoninae). - Ibid. 131 : 289-302. & B. Regner Nielsen, 1971 : Species differences in ovipository instincts within the Verrnile- oninae (Diptera Brachycera, Rhagionidae = Leptidae). - Ibid. 134: 149-203. Wigglesvrorth, V. B., 1953: The principles of insect physiology. - London. Youthed, G. J. & V. C. Moran, 1969: Pit construction by rnyrrneleontid larvae. - J. Insect Physiol. 15: 867-875.