192 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



embryological study is a burrowing May fly. The mandibular tusk, 

 which he calls the "morphological equivalent of a mandibular palp," is a 

 secondary modification appearing at the time of differentiation of the 

 canines and molar region. It is lateral in origin but arises from the outer 

 apical region. 



ODONATA 



Dragonflies and Damsel Flies 

 The eggs of Lihellula pulchella, Erythemis simplicicollis, and Plathemis 

 lydia, the three species of Odonata studied by the junior author, vary 



Fig. 113. — Lihellula. 



A-C, longitudinal section, successive stages. 

 {nu) Nucleus. 



slightly in shape and size, but their development is similar in ah respects. 

 They are laid in the same localities, and the actions of the female during 

 oviposition are similar. The females of all three species lay the eggs as 

 they fly in circles over the water and dip their abdomens momentarily to 

 wash off the egg mass being extruded from the oviduct. The eggs of the 

 mass sink to the bottom singly, and each soon produces a viscid substance 

 which causes the egg to stick to anything it touches. Sometimes the eggs 

 drift into a clump and become deeply imbedded in a mass of jelly. The 

 eggs of Plathemis lydia are ovoid in form but are pointed at both ends. 

 There is no difference between dorsal and ventral sides. The eggs of 

 Lihellula 'pulchella and Erythemis simplicicollis are approximately the 

 same size but are smaller than those of Plathemis and are not so pointed. 

 The shell of all three is tough and creamy white; the yolk is very 

 dense; and the globules often appear cuboidal or hexagonal in cross sec- 

 tion. The peripheral cytoplasm, which is very thin, adheres so closely to 

 the chorion that a thin vitelline membrane reported in other species was 

 not seen. Maturation and fertilization were not observed. In the egg of 

 Lihellula, three hours old, a nucleus is present at the center of the yolk, 



