viii AETHEOPODA 245 



INSECTA 



Classification adopted (only those orders specially alluded to 

 in the text are mentioned] 



rThysanura 



Aptera { ~ , , 



\ Collembola 



(Orthoptera 



.... Paraneuroptera (Odonata) 

 Hemimetabola XT,, 



Ephemeroptera 



[Hemiptera 

 ( Coleoptera 



TT i j. v i Lepidoptera 

 Holometabola i 



Hymenoptera 



IDiptera 



The Insecta, even if we confine ourselves to Insecta Hexapoda 

 and exclude Myriapoda, are an enormous group, including nearly 

 three hundred thousand named species. A large amount of work has 

 been done on their development since the earliest days of scientific 

 embryology, and a full discussion of this would lead us entirely 

 beyond the limits assigned to this work. Fortunately, in compara- 

 tively recent times the embryology of two species has been worked 

 out in a thoroughly satisfactory manner, viz. that of Doryphora 

 (Leptinotarsa) decemlineata, the so-called potato bug, or Colorado 

 beetle, by Wheeler (1889), and that of Donacia crassipes, by 

 Hirschler (1909), a beetle belonging to a closely allied family and 

 abundant throughout Europe. The results of these two investigators 

 are in agreement in all important points, but as Hirschler's work 

 is the most recent, we shall select Donacia and not Doryphora as 

 type. 



DONACIA 



The eggs of both forms are laid in batches enclosed in a cocoon 

 and attached to the under surface of leaves, those of Doryphora to 

 the leaves of the potato plant and allied forms, those of Donacia to 

 the leaves of water-plants. In studying the development of Donacia 

 Hirschler punctured each individual egg with a fine needle, whilst 

 observing the whole cocoon under a powerful dissecting microscope. 

 He then immersed the cocoon for two to three hours in a mixture of 

 equal parts of 3 per cent aqueous solution of HN0 3 , and concentrated 

 aqueous solution of corrosive sublimate. The cocoons were then 

 passed up through grades of alcohol till they had reached that of 

 90 per cent, in which they remained for twenty-four hours. 



Then the cocoon was cut into pieces, each of which contained 

 eight or ten eggs. These pieces were stained for twenty-four hours 

 in a half per cent watery solution of thionin, the stain being 

 differentiated by subsequent immersion for twenty-four hours in 



