130 ENTOMOLOGY 



The number of eggs laid by one female differs greatly in different 

 species and varies considerably in different individuals of the same species. 

 Some of the fossorial wasps and bees lay only a dozen or so and some 

 grasshoppers two or three dozen, while a queen honey bee may lay a 

 million. Two females of the beetle Prionus laticollis had, respectively, 

 332 and 597 eggs in the abdomen (Mann). A. A. Girault gives the fol- 

 lowing numbers of eggs per female, from an examination of twenty egg- 

 masses of each species: 



Maximum. Minimum. Average. 



Thyridopteryx ephemerceformis 1076 753 941 



Clisiocampa americana 466 313 375-5 



Chionaspis furfura 84 33 66.5 



Hatching. Many larvae, caterpillars for example, simply eat their 

 way out of the egg-shell. Some maggots rupture the shell by contortions 

 of the body. Some larvae have special organs for opening the shell; 

 thus the grub of the Colorado potato beetle has three pairs of hatching 

 spines on its body (Wheeler) and the larval flea has on its head a tempo- 

 rary knife-like egg-opener (Packard). The process of hatching varies 

 greatly according to the species, but has received very little attention. 



Larva. Although larvae, generally speaking, differ from one another 

 much less than their imagines do, they are easily referable to their orders 

 and usually present specific differences. Larvae that display individual 

 adaptive characters of a positive kind (Lepidoptera, for example) are 

 easy to place, but larvae with negative adaptive characters (many Dip- 

 tera and Hymenoptera) are often hard to identify. 



Thysanuriform Larvae. Two types of larvae have been recognized 

 by Brauer, Packard and other authorities : thysanuriform and cruciform; 

 respectively generalized and specialized in their organization. The 

 former term is applied to many larvae and nymphs (Fig. 211, C, D) on 

 account of their resemblance to Thysanura, of which Campodea and 

 Lepisma are types. The resemblance lies chiefly in the flattened form, 

 hard plates, long legs and antennae, caudal cerci, well-developed mandib- 

 ulate mouth parts, and active habits, with the accompanying sensory 

 specializations. These characteristics are permanent in Thysanura, but 

 only temporary in metamorphic insects, and their occurrence in the latter 

 forms may properly be taken to indicate that these insects have been 

 derived from ancestors which were much like Thysanura. 



Thysanuriform characters are most pronounced in nymphs of Blat- 

 tidae, Forficulidae, Perlidae, Ephemeridae and Odonata, but occur also in 

 the larvae of some Neuroptera (Mantis pa) and Coleoptera (Carabidae 



