199 ON THE LIFE HISTORY OF BUCENTES (SIPHONA) GENICULATA (DIPTERA: TACHINIDAE), PARASITE OF TIPULA PALUDOSA (DIPTERA) AND OTHER SPECIES1. BY JOHN RENNIE, D.Sc. AND CHRISTINA H. SUTHERLAND, M.A., B.Sc, Carnegie Scholar. (From the Laboratory of Parasitology, University of Aberdeen.) (With Plate XIV.) THE Dipterous Family, Tachinidae, comprises a very large number of species whose larvae live as parasites within other insects, particularly in their larval forms. Bucentes (Siphona) geniculata, one of the Tachinidae whose larvae are considered in the following paper, is a small ordinary looking fly, blackish in eolour and showing a somewhat greyish abdomen, with prominent abdominal bristles. The labium is long and slender, sharply geniculated about the middle of its length, and folded like a clasp knife under the head when not in use2. In a former paper (1912) one of us has recorded the occurrence of the larva of this species as a parasite in the body cavity of Tipula larvae. It is probably not confined to one host species. In one collection of Tipula larvae, a pro- portion of which yielded the parasite, the majority of the survivors, on hatching, proved to be Tipula oleracea. In most instances, however, we have found the infected insects to be T. paludosa. Since the original observation in 1912, we have found this larva regularly every year, and consequently regard it as a normal parasite of Tipula. Other observers record it from Mamestra brassicae and a related species, Siphona cristata, is reported by Roubaud (1906) to occur in Tipula gigantea. OCCURRENCE AND DISTRIBUTION OF THE ADULT PLY. From an examination of records relating to the adult insect, kindly sub- mitted to us by the entomologists in charge at the British Museum and the Royal Scottish Museum, we find the fly is widely distributed throughout 1 This work has been carried out with the aid of Grants from the Board of Agriculture for Scotland, to whom we desire to express our thanks. 2 A full description of the structure and habits of the adult fly is reserved for a further paper 200 Bucentes (Siphona) geniculata Great Britain. It is recorded from Devon, Hereford, Hants, Kent, Radnor, Middlesex, Hampshire and Inverness-shire. We have found it in Northumber- land and Aberdeenshire. The adults of this fly are recorded over the period from May till Sep- tember. Schiner (1862) writes regarding the genus: " The flies live in dry places especially heaths. They are also found on Umbelliferae and on waste ground are often to be seen in large crowds on Daucus carota." He also states that B. geniculata visits Erica vulgaris. Walker (1853) reports these flies as "very common." The following list of flowers known to be visited by B. geniculata is com- piled from Knuth's Handbook of Flower Pollination (1909). Viola lutea. Valerianella olitoria. Stellaria holostea. Succisa pratensis. Medicago lupulina. Eupatoria cinnabinum. Potentilla sylvestris. Pulicaria dysenterica. Potentilla sterilis. Myosotis sylvatica. Damns carota. Mentha aquatica. Hedera helix. Origanum vulgare. Asperula cynanchica. Hottonia palustris. Asperula odorata. METHODS OF INVESTIGATION. The Tipula larvae, after being washed free of soil, were examined individu- ally and with a little practice the parasitic maggot could be readily recognised beneath the skin of the host larva as an elongated yellowish patch of definite shape. The proportions found to be infected varied a good deal in different localities and in different years. In one area, viz. Echt, Aberdeenshire, the numbers reached a significantly high figure. Over a period of one month, during which larvae were examined, a progressive decrease in the percentage of infected individuals was noted. This was found to be due to the fact that as the season advanced, the infected larvae were dying off, and pupae of Bucentes were becoming increasingly more numerous in the soil. It must therefore be borne in mind that for the first generation, unless examinations are made not later than about February, any estimate of the degree of infection must be below that actually existing. The following data illustrate this: J J. RBNNIB AND 0. H. SUTHERLAND 201 LARVAE COLLECTED IN ECHT AND LOWER DEESIDE AREAS. Data for 1918. Infected Healthy Percentage Lot Date larvae larvae Total infected 1. Feb. 20 170 434 604 28-1 2. „ 27 127 243 370 34-3 3. ,. 28 164 524 688 23-8 4. Mar. 1 73 386 459 160 5. 6 103 442 545 190 6. „ 7 182 819 1001 18-2 7. „ 11 56 345 401 13-7 8. „ 16 42 258 300 140 9. „ 23 79 221 300 26-3 Totals 996 3672 4668 21-3 LARVAE COLLECTED IN NORTHUMBERLAND. For purposes of comparison we obtained a lot of larvae from the north of England. These yielded the following result: Infected Healthy Percentage Date larvae larvae Total infected Mar. 1 51 794 845 6-0 In other areas where larvae were collected in fewer numbers, the parasite was also found and all our observations suggest that it is relatively abundant. During the season 1919, Tipula larvae were again collected and further data concerning the occurrence of Bucentes parasites were obtained. Search was made for Tipula larvae in fields on the College of Agriculture experimental farm at Craibstone, Aberdeenshire, from the month of December onwards. During December and January only very small numbers of Tipula larvae were found and of these none were infected with Bucentes. In February and March the numbers found increased slightly and out of 56 larvae, three were found infected, one of these harbouring four parasitic maggots. From April onwards appreciable numbers were obtained; the data for the season are shown in the table given below: LARVAE COLLECTED IN LOWER DEESIDE AND LOWER DONSIDE. Data for 1919. Infected HealthjL Percentage Lot Date larvae larvae Total infected Craibstone. 1. April 7 25 349 374 6-6 2. 11 39 237 276 141 3. 22 56 429 485 11-5 4- 25 46 288 334 13-4 5. May 8 1 155 156 0-8 No infected larvae obtained here during period May 9 to June 20. 202 Bucentes (Siphona) geniculata LARVAE COLLECTED IN LOWER DEESIDE AND LOWER DONSIDE (continued). Data for 1919. Infected Healthy Percentage Lot Date larvae larvae Total infected Craibatone. 6. June 21 13 66 79 16-45 7. „ 25 23 30 60 38-6 (+7 otherwise infected) 8. ., 30 22 49 77 28-5 (+ 6 otherwise infected) 9. July 7 6 9 15 400 Anguston, Culler. 10. April 26 1158 3467 4625 250 11. May 2 107 456 563 190 12. 9 50 903 953 5-2 13. .. 16 3 807 810 0-37 1562 7245 8807 17-6 As is indicated in the foregoing table no infected larvae were obtained between May 16th and June 21st. This was the case in other areas in the north of Scotland of which several were examined. The fall to 0-6 per cent, at the beginning of May, and the complete absence of infected larvae in the soil for nearly five weeks, we attribute to the fact that the Bucentes maggots had pupated, and thus the Tipula larvae then found were such as had escaped infection by the parasite. Larvae obtained at Craibstone on June 21st showed an appreciable per- centage infected by Bucentes larvae, and from that date until July 7th, when the oats had grown too high to allow of further search for grubs, infected larvae continued to be obtained in high percentages, as is shown in the table. NUMBER OF PARASITIC LARVAE PER HOST. Observations made in 1918 show that one, two, three, and even four parasitic maggots may occur in one host. This was also the case in specimens found in the early months of 1919. A noticeable feature among the larvae found in the later part of 1919, viz. from June 21st onwards, was that there were seldom fewer than two Bucentes maggots in each host larva, 5-6 being common, and in one case as many as nine Bucentes maggots were found in a single host. SIZE OF PARASITIC LARVAE FOUND. Larvae from 1-5 mm. up to 8-5 mm., which represents the maximum size, have been found. Even within the same host, the larvae may differ in size. For example, two maggots, 5 mm. in length, were found in the same host along with two others of 3 mm. In one case, where nine parasites occurred together, these were all 3 mm. in length. It seems more probable to us that these differences J. RENNIE AND C. H. SUTHERLAND 203 in size in parasites occurring together are due to variations in the rate of growth than that the same host should have been parasitised on two or more separate occasions. DURATION OF LARVAL PERIOD. First Generation. In the season 1918, Bucentes larvae were obtained as early as February 20th. These larvae were on an average about 4 or 5 mm. in length at this date. In 1919, these larvae were first obtained on February 28th; they were from 3 to 5 mm. in length. This suggests that the parasitic larvae found in February are the product of eggs laid the previous autumn, the larvae having hibernated in the host. Second Generation. From June 21st onwards, large percentages of the collected host larvae were found to be infected with Bucentes, whose maggots were of such a size, viz. 1-3 mm., as to lead to the suspicion that the Tipula larvae had been recently infected. From these Bucentes maggots a second generation of flies was bred which began to emerge on July 25th. An earlier generation of flies was bred out in May and the early part of June and by June 13th all these flies had died.