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TEXT-BOOK OF ENTOMOLOGY 



Attelabus, a weevil, as we have observed, the primitive band rests on the 

 outside of the yolk, but in Telephorus fraxini it is immersed. In the Hemiptera 



FIG. 515. Ventral view of five developmental stages of Hydrophilus : a and b, places at which 

 the blastppore contracts ; af, edge of the amnion-fold ; /', caudal fold ; ./", paired head-fold of 

 the amnion ; an, antenna; en, last segment; g, pit-like invagination (first indication of the amni- 

 otic cavity) ; k, head-lobes ; r, furrow-like invagination ; s, portion of the primitive streak covered 

 by the amnion. After Heider, from Lang. 



it is immersed (Fig. 516), but there is much variation in this respect, the degree 

 of immersion being most marked in the Coccidse (Aspidiotus), and least so in 

 Corixa. Besides the position of the primitive band, 

 there are in Odonata and Hemiptera differences in 

 the origin of the primitive band itself and of the 

 embryonic membranes. 



Korschelt and Heider divide the early embryo of 

 insects into t two types : 



1. Into those with a superficial primitive band ; 

 viz., where there is no passage of yolk-elements into 

 the space between the amnion and serosa. The 

 primitive band has in such cases a relatively super- 

 ficial position (Figs. 508, 509, 521, 535). Examples 

 are certain Orthoptera (Blatta, CEcanthus, Mantis, 

 Gryllotalpa) , also certain Hemiptera (Corixa), cer- 

 tain Coleoptera, and the Trichoptera, Diptera, and 

 Hymenoptera. 



2. Into those with an immersed primitive band, 

 with the space between the serosa and amnion 

 filled with yolk (Figs. 517, 618, 534). Examples 

 are the orthopterous Stenobothrus, Odonata, many 

 Hemiptera (the Pediculina and Pyrrhocoris), the 

 Coleoptera already mentioned, and Lepidoptera. 



It should be observed, however, that these differ- 

 ences are of little phylogenetic or taxonomic value, 

 since genera of the same order, notably the Coleop- 

 tera, differ as to the position of the primitive band, 

 so also two orders so nearly allied as the Trichoptera 

 and Lepidoptera. 



Differences between the invaginated and overgrown primitive band. In 

 respect to the mode of origin of the primitive band and its relative position, 



'HI 



Fu;. 516. Embryo of the 

 louse : am, serosa ; db, amnion ; 

 as, antenna ; vk, clypeus. After 

 Melnikow. 



