266 



INVERTEBRATA 



CHAP. 



ser 



The remarkable agreement in all essential details between the 

 development of a primitive insect like Lepisma and a highly modified 

 one like Donacia, which is disclosed by Heymons' researches, enables 

 one to pass over the embryonic development of other groups quickly. 

 In Odonata (Paraneuroptera) the embryonic area is comparatively 

 short, and the hinder end is invaginated into the interior of the egg ; 



or to put it in another way, 

 there is only a posterior, no 

 anterior amniotic fold ; but 

 in all the higher insects, as 

 in Donacia, there are both 

 anterior and posterior am- 

 niotic folds, and this is the 

 case in true Orthoptera. 



In the Lepidoptera yolk 

 spheres pass in between 

 amnion and serosa after the 

 front and back amnion folds 

 have united, so that the 

 embryonic area is said to be 

 immersed. In Lepidoptera, 

 too, all the abdominal seg- 

 ments develop appendages, 

 and these are retained on 

 those segments which, in 

 the caterpillar larva, possess 

 sucker feet. 



In those Hymenoptera 

 which possess a caterpillar- 

 like larva, the same thing is 



FIG. 211. Two diagrammatic lateral views through true. It IS true also of 



the egg of Machilis alternata in different stages man y Coleopterous embryos, 



of development. (After Heymons.) ^ Melolontha. In these 



A stage of incipient imagination of the embryo. larvae _ in early st ageS all 



B, stage of more complete mvagination of the embryo. J 



Letters as in previous figure. the abdominal segments 



have appendages, but all 



disappear except that on the first abdominal segment, which lasts a 

 long time and becomes glandular. 



The eggs of Diptera, which develop very rapidly, show a 

 comparatively long embryonic area, and the same is true of the 

 rapidly developing parthenogeuetic eggs of Hemiptera. In fact, the 

 variation in relative size of embryonic area and yolk seems to be 

 the most important feature in the eggs of different kinds of insects. 



ser 



METAMORPHOSIS 



We shall now consider the second stage of the development of 

 insects, viz. the metamorphosis of the larva into the adult form. The 



