Populations of the Wartbiter to Biotope

Dimensions in two coastal populations of the wartbiter (Decticus verrucivorus (L.)) in relation to biotope R.T. Tienstra P.G. Otterweg 5 8549 EV Luinjeberd Samenvatting biotope (see De Smidt, 1981), as opposed to the greater In een vorig artikel (Tientra, 1992a) heb ik de kleurvaria- variability of the Thy animals, caused by a much more and former tie binnen de wrattenbijterpopulaties van het Gooi en Thy varied biotope, consisting of heathland, dunes (Denemarken) besproken. agricultural vegetation. will the differences in dimensions of Hier komt het verschil in afmetingen tussen dieren van In this article I treat deze locaties aan de orde. the animals from these two areas. De Gooise wrattenbijters hebben minimale afmetingen (tabel 1). Dit kan te maken hebben met het ontbreken van 1. Size I voldoende eiwitrijke voeding in de arme Genista anglicae The Gooi wartbiters were extremely small (Table 1). their - Callunetum vegetatie, de door eeuwenlange begrazing consider this to be another result of differences in met schaapskudden ontstane heide-akker, die in het Gooi biotope. The larvae of Decticus verrucivorus are probably the in de jaren ’60, vóórdat vergrassing en verbossing toesloe- predominanüy camivorous (Cherrill, 1989); adults, habitat of are herbivorous: in gen, de belangrijkste was van de volwassen dieren. probably out sheer necessity, capti- brunneus and Op Thy bevinden de dieren zich in een zeer gevarieerd vity they immediately hunt for Chorthippus biotoop en ze hebben dan ook flinke afmetingen. De die- C. albomarginatus, and this from the very start of their ren van de heidehabitatlijken hier eveneens iets kleiner te captivity. In their natural habitat in the Gooi area, a dry zijn (tabel 1). De vleugels van de Thy’se wrattenbijters Callunetum, I did observe them eating only the growing zijn flink ontwikkeld, ze vliegen van de hoge duinen naar tips of the heather plants; their faeces were invariably dry, beneden. In dit heuvelachtige gebied zullen dieren met fibrous and greenish. of animal food is absent lange vleugels enig selektief voordeel hebben, doordat ze Undoubtedly a sufficiënt quantity ook op deze wijze naar andere baltsplaatsen kunnen gaan. in the Callunetum typicum during late August and September, when Decticus needs many proteins for its Introduction progeny, as this period has the highest reproduction rale. The Gooi wartbiters - and to a lesser extern those from the In a previous article (Tienstra, 1992a) 1 have compared heathland habitats of Thy, although not the Thy animals the colour variability of wartbiters in the Gooi area from the “green” habitats (in the middleof August in the (Holland) and at Thy (Denmark). I have put forward the dry 1992 summer (Tienstra, 1992a))- had very lean appe- believe hypothesis that the preponderance of the dark blue-green arance, with shrunk abdomen. This leads me to colour of Gooi animals the was due to the uniform anthro- that the heathland habitat, especially the dry Callunetum Genisto - pogenic scanty anglicae Callunetum heathland typicum of the Gooi area, is too scant for the warlbiter. NieuwsbriefSaltabel 9(1993) 13 males hind lemur prone turn elytron 1 Harz 25.3-35.0 6.9-10.5 21,3-250 liiiii 'tGooi 26.3 7.6 24.7 10 (1915-1967)2 (25.0-28.6) (6.5-89) (20.8-27.3) heath-habitat Thy (1992)2 28.5 7.8 27.9 4 heath-habitat (28.0-29.0) (70-8.5) (27.0-29.0) rir\ q Thy (1992) 31.7 8.7 13 green-habitat (28-34) (8.0-90) 23.0-34.0) females hind femur pronotum elytron ovipositor n Harz 28.0-37.0 7.7-11.0 23.0-30.0(38,0) 170-26,0 t Gooi 28.3 7,7 23.1 18.3 10 (1940-1967) (26.0-32.6) (6.S-8.9) (21.0-27,8) (15,9-21.0) heath-habitat ■.. ■ Thy (1992) 32.0 8.5 28.5 225 , heath-habitat Thy (1992) 37.0 9.0 34,0 37.0 1 green-habitat ’Harz, 1960 «Tienstra, 1992b STienstra, 1992a Table 1 Moasures in two Coastal populabonsof Decticus verrucivorus (L) and in literature (in mm) the male wartbiter the As well as other bushcrickeis offers a although possibility of parental investment cannot nupüal present to the female before copuiation; a sper- completely be excluded. The spermatophylax size per little with failed effect female matophore, containing two ampullae sperm, se to show any on reproduction. in The it found that the volume of together with a spermatophylax, rich proteins. Instead, was ejaculate trans- spermatophore is attached to the abdomen of the female ferred to the female is the factor controlling female and the transfer the into the female and behaviour. Thus the main assures of sperm receptivity oviposition body while she is eating the spermatophylax. Moreover, function of nuptial feeding may be defined in this spe- the female will not copulate with another male during cies as sperm protection. this time. Experimenls with doubiy mated females (Wedell, 1991) A very large spermatophylax, such as of the bushcricket strongly suggest that sperm mixing occurs within the Ruspolia verticalis, may function as a parental invest- wartbiter, numerical sperm competition being the acting of Fertilization ment, favouring the survival rate the progeny by mechanism. success seems to be directly increased fertility (Gwynne, 1984 in Wedell, 1991), as related to the size of the spermatophylax, which in turn 1988 well as by larger eggs, larvae and adults (Gwynne, is correlated to the volume of ejaculate transferred. in when the Gwynne, 1990). Thus, second male provides the female with a Wedell and Arak (1989) however have proven that the larger spermatophylax relative to the first male, his spermatophylax of Decticus verrucivorus has the right volume of ejaculate will be larger and there will be a size for the transfer of the ejaculate to be accomplished. numerical superiority of his sperm. Hormones in the 14 NieuwsbriefSaltabel 9 (1993) ejaculate may cause the postmating unreceptivity period These circumstances suggest a limited fecundity and a of and increase restricted of the female, hasten the onset oviposition copulation frequency. Moreover, they may have the rate of egglaying (Wedell & Arak, 1989). caused a decline in genetic diversity within the Gooi pop- in Cherrill & Brown (1990) report dramatic increases ulations, as doublé matings hardly occurred, especially in weight of adult females during the month of August This the preceding three decades, when population densities is probably caused by the maturationof the eggs and the feil dramatically. accumulation of body fat At the end of August the females attain their maximum body weight, so that they should in until in Table 2 not reach a peak potential fecundity early provides data on the Gooi populations upto their September (Sussex population, 1987and 1988). In extinction in 1989. During the mid 1960’s the populations addition, Cherrill & Brown state that the weight increase had already been reduced, especially on the South Heath, of the male Decticus after larval development is bigger were some 20 males were divided over 5 leks occupying than can be accounted for by the production of a single an area of 2 sq. kms. In fact every chirping male could be that the males considered On spermatophore. Therefore they suggest to be an isolated one. the North Heath 2 of accumulate sufficiënt material enable them the 5 clusters of animals to to produce were not yet desintegrated. a numberof spermatophores in rapid succession. The situation can be regarded as an example for other However, I doubt if the mechanisms described will often heathland areas in our country. Though these biotopes occur in the field in the Gooi biotope: if the summer is dry were probably not that scant - the wartbiter specimens the second halfof and in - is collected from heathland in the central during August September as areas part of our often the case - and, if not, the weather may be so bad in country do not indicate such small size as those from the Atlantic climate exclude all - this northern as to nearly Gooi area (University Museum of Amsterdam) the wart- copulation opportunities, it may be virtually impossible biter was already extinct in the 1960’s in most localities for the wartbiter to find enough proteins at all. Cynical where it had previously been known. A possible explana- of wartbiter’s coincidence: this period the highest repro- tion for the longer survival of the Gooi populations may duction rate is the summer holiday period in Holland; in be the continuous presence of flocks of sheep until 1966, the urban Gooi 30 km. of i.e. decades later densily populated area, east two than elsewere. In this case, recre- conditiosine for the wartbiter. ation in the urbanised Gooi Amsterdam, not a cure had a favourable effect! The the of the sheep prevented expansion grass Deschampsia flexuosa and the germination of foliage trees, especially of the agressive exotic Prunus serotina (see also Tienstra, South Heath (5 leks) North Heath (5 leks) 11 August 1966 29 15August 1966 62 1992a). 16 August 1966 62 At the moment, three very small populations of wartbiter 21 August 1967 22 22 August 1967 51 still in the is 23 August 1967 iii; occur Netherlands, of which only one more 9 1969 22 9 1969 iiii August August less or subject to any conservancy management. 10 August 1969 llll the Gooi Summarizing, scanty habitat may have been the 31 1988 0?* 31 1988 iiü August August clue to the small dimensions of the wartbiters and perhaps 22 August 1989 0! 22 August 1989 0! also the the to greater vulnerability of com- 13 August 1991 0* 13 August 1991 iiîi populations, pared to those of e.g.

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