Physiological Entomology (1992) 17, 255-259. Initial reproductive investment and parental body size in. Cryptocercus punctulatus (Dictyoptera: Cryptocercidae).
Physiological Entomology (1992) 17, 255-259
Initial reproductive investment and parental body size in Cryptocercus punctulatus (Dictyoptera: Cryptocercidae) CHRISTINE A . NALEPA and D O N A L D E . MULLINS” Department of Entomology, North Carolina State University, Raleigh, N.C., and “Department of Entomology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, U.S.A.
Abstract. Females of the subsocial woodroach Cryptocercus punctulatus Scudder generally have a single oviposition period during which they produce one to four oothecae. Monogamous pairs and their recently deposited oothecae were collected in the field, and measured, weighed and analysed for nitrogen in the laboratory. Females put 9.8?2.4% (mean? SD) of their dry weight and 11.6 2 0.5% of their body nitrogen into oothecae. The total nitrogen and dry weight of the brood were positively related to the post-oviposition total nitrogen and dry weight of the mother. A female’s investment, on a per nymph basis, averaged 0.06% of her post-oviposition dry weight. Females may be able to recover up to 58.7% of the nitrogen invested into a brood by consuming the egg cases after hatch. Overall, there was little variation in the width of head capsules of adults in this species, and this parameter was not significantly different between the sexes. Females were heavier than males (dry weight) ( P = 0.06). Within pairs, the weights and nitrogen contents of males and females were positively correlated, probably because they feed together in the same log for nearly a year prior to reproduction. Key words. Cryptocercus, cockroach, reproductive investment, eggs, body size, weight, nitrogen, sexual dimorphism.
Introduction Females of the wood-feeding cockroach Cryptocercus punctulatus Scudder generally have a single oviposition period in which they produce from one to four oothecae containing a mean of 7 3 eggs (Nalepa, 1988a). After the hatch of this brood, adults exhibit a period of parental care that lasts 3 or more years (Seelinger & Seelinger, 1983; Nalepa, 1984). Because it restricts further reproduction, this intense brood care may explain the apparent semelparity of this species (i.e. one brood per lifetime) (Nalepa, 1988b). The goal of the present study was to test if the eggs laid by C.punctulatus represent a substantial investment of a female’s body weight and body nitrogen, and if clutch size varies with parental size. Because this species is monogamous, the male shares in parental duties, and the male and female mate repeatedly during the course of their
Correspondence: Dr Christine A. Nalcpa, Entomology Department, Box 7613, N.C. State University, Raleigh. N.C. 27695-7613, U.S.A.
association (Nalepa, 1984, 1988a), the size of the male in reproducing pairs was also measured. Nitrogen content was chosen as one indication of investment because dietary nitrogen is known to influence fecundity in cockroaches (reviewed by Mullins et al., 1991). Furthermore, nitrogen is particularly low in the diet of wood-feeding insects and may, therefore, be one of the principle selective pressures shaping the life history of Cpunctulatus (Nalepa, 1984, 1988b).
Materials and Methods Pairs of adult C.punctulatus and their oothecae were collected shortly after their oviposition period from rotting logs on the grounds of Mountain Lake Biological Station, Giles County, Virginia, 29 June to 2 July 1984 and 26 June to 5 July 1986 using methods previously described (Nalepa, 1984, 1988a). Oothecae were analysed individually in 1984 ( n = 57 oothecae from twenty-two females, range one to four oothecae per female); all oothecae from one brood were pooled for the 1986 study ( n = 13). A total of thirtyfive pairs with their broods were analysed. 255
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Chriaitte A . Nalepa and Donald E. Mulliris
Three measurements were taken to determine adult size: wet weight, dry weight. and width of the head capsule. Weights were determined on a Cahn Electrobalance Model 7500, and head capsules were measured using an ocular micrometer on a dissecting microscope. Oothecal size was determined by wet weight, dry weight and egg number. Wet weights and head capsule sizes were taken on samples collected in 1984 only. In 1986, oothecae from an additional six females were isolated in glass shell vials and checked daily for hatch. Neonates that emerged from these oothecae were analysed for dry weight and total nitrogen; two to four nymphs were pooled per sample. Nitrogen was measured using micro-Kjeldahl digestion (Schmidt, 1961; Umbreit et al., 1957). All material was frozen until analysis, lyophilysed, then pulverized in a dental amalgamator. Three replicates of each sample were analysed, and the results averaged. Optical densities were read on a Perkin-Elmer Lambda 3B UV/VIS spectrophotometer at 490mp. All values are reported as the mean tstandard deviation. Data were analysed using analysis of variance and Pearson correlation; paired comparisons were made using the [-test (SAS User's Guide: Statistics, 1985, SAS Institute, Cary, N.C.)
Broods (pooled ootlzecae of a single female)
The positive relationship between egg number and total nitrogen also held true at the level of the entire brood. Nitrogen investment by a female, then, could be predicted1 from the number of eggs oviposited (Fig. 1). Females put 6.1 t 1.9% of their wet weight ( n =22): 9.8 z 2.4% of their dry weight ( n = 35) and 11.6 i 2.9% ( n = 35) of their body nitrogen into oothecae. The totall nitrogen and dry weight of a brood were positively relatedl to the post-oviposition total nitrogen and dry weight of the mother (r2=0.31, P
