200 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



stom 





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ri.jy_ 



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ry 



IM. 



•ysp 



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that they had while in the egg. The labium at this time is long and 

 slender and lies extended back on the ventral side under the thorax and 

 abdomen. For a short time after emergence the nymph lies quietly on its 

 side or back, but soon the embryonic integument breaks and the nymph 

 emerges. When it is free, the tip of the labium 

 assumes a position in front of the head like a 

 mask. Although the time varies between hatch- 

 ing and completion of the first molt, the whole 

 procedure takes less than ten minutes. 



Immediately after hatching, the tubular mid- 

 gut extends to the second or third segment of 

 the abdomen (Fig. 125). Soon after the nymph 

 is free of the embryonic exuviae, the mid-gut 

 shortens by nearly one-half its previous length 

 (Fig. 126). For the first day or two, and 

 occasionally for as much as three days, the larva 

 does not take food. Even after the larva first 

 begins feeding, it is apparent that the digestive 

 epithehum has not yet formed. The mid-gut 

 still contains in its walls vacuolate tissue charac- 

 teristic of the embryonic stages, the food being 

 stored in the posterior end of the fore-gut. 



Tschuproff, working with Epitheca bimaculata 

 and Libellula quadrimaculata, stated that the 

 anterior and posterior thirds of the mid-gut are 

 formed from ectoderm cells which grow out cup- 

 like around the yolk from the stomodaeal and 

 proctodaeal invaginations. The middle section, 

 on the other hand, is formed from entodermal 

 cells within the yolk. These cells migrate to the 

 periphery of the mid-gut where they form re- 

 generative cell masses lying in crypts adjacent to 

 the muscle layer and between the yolk compartments. When the mid- 

 intestine first forms in Plathemis, the whole wall is made up of yolk com- 

 partments. The ectodermal end of the stomodaeum forms a bulbous 

 invagination corresponding to the stomodaeal valve of other insects; the 

 proctodaeum is restricted to a small ring around the anterior end of the 

 hind-gut. 



From a study of sections of Plathemis, Erythemis, and Libellula it 

 .seems justifiable to assume that ectodermal cells do not migrate from the 

 stomodaeum and proctodaeum but that the definitive digestive epithelium 

 replaces the yolk compartments progressively from both ends, the process 

 being prolonged until nearly time for the molt at the end of the first 





W 



proct 



Fig. 124.— Plathemis. 

 Mid-gut of 14-day embryo. 

 Qu) Lumen, (proct) Proc- 

 todaeum. {stom) Stomo- 

 daeum. (wo) Outer wall 

 of yolk spherule, (ws) 

 Side wall of yolk spherule 

 (ysp) . 



