520 TEXT-BOOK OF ENTOMOLOGY 



of the three pairs of palpi which guard the vaginal opening is registered in an 

 exquisite pattern on the inner face of either half of the crista." l 



The mode of oviposition in the locust has been fully described by 

 Riley, who states that the eggs pass down and out of the oviduct, 

 and "guided by a little finger-like style" (Fig. 298), they pass in 

 between the horny valves of the ovipositor, and issue at their tips 

 amid the mucous fluid which forms the egg-capsule (Fig. 492). 



Vitality of eggs. It is well known that the eggs of phyllopod 

 and other fresh-water Crustacea have wonderful vitality, withstand- 

 ing extreme dryness for several years, at least from two to ten. 

 Such cases are unknown among insects. It has been observed, how- 

 ever, by T. W. Brigham, and also by L. Trouvelot, that the eggs of 

 the walking-stick (Diapheromera femorata) for the most part hatch 

 only after the interval of two years. 2 



The eggs of Bittacus are said by Brauer to lie over unhatched for 

 two years ; indeed, the first condition of their hatching is a complete 

 drying of the earth in which the eggs lie, the second is a succeeding 

 thorough wetting of the ground in spring. 



Appearance and structure of the ripe egg. - - The eggs of insects are 

 on the whole rather large in proportion to the size of the parent, 

 especially so in many minute forms, as the fleas, lice, etc. 



Their general shape is spherical or oval, often cylindrical ; where 

 the eggs are long and cylindrical a dorsal and ventral side can be 

 distinguished (Fig. 502). They are in the Tortricidae and Limacodid 

 moths flattened, thin, and scale-like. In the eggs of locusts and 

 grasshoppers, as well as certain Diptera, the ventral side of the 

 embryo corresponds to the convex side, and the concave side of the 

 egg to the dorsal region of the embryo (Figs. 502 and 493). 



There is an anterior and posterior end or pole, the anterior end 

 being that which in the body of the parent lies towards her head, or 

 towards the upper or distal end of the ovarian tube. Towards this 

 end lies in the later stages of embryonic life the head-end of the 

 embryo, while the posterior end of the embryo is turned towards the 

 hinder pole of the egg (Figs. 493 and 520). 



The egg-shell and yolk-membrane. -- The ripe egg is protected by 

 two membranes : 1, an inner or vitelline membrane or oolemma (dli) 

 (Fig. 500, cl), produced in the egg by a hardening of the outer layer, 

 and 2, the outer or chorion (c), which is secreted by the cells of the 

 ovarian follicle. The latter is divided into two layers : an inner, 

 the endochorion, and an outer, the exochorion. 



1 Journ. Morph., iii, Boston, pp. 209, 300. 



2 Proc. Boston Soc. Nat. Hist., xi, pp, 88, 89. 



