6 BRITISH APHIDES. 



insect something like Libellula was the type of the 

 original stock. 



Sir J. Lubbock suggested, as a part solution of these 

 difficulties, that there is a distinction between develop- 

 mental and adaptive changes. Metamorphic variations, 

 indeed, do not necessarily proceed from the simple to 

 the complex, like ordinary growth of tissue; but 

 organs increase or become atrophied, according to the 

 wants of the insects attending their conversion into 

 pupae or images. 



There can be little doubt that insects undergo 

 minute modification of form and organs throughout 

 their entire existence, if we except, perhaps, the 

 quiescent periods of chrysalis life. The action of tem- 

 perature, food, climate, starvation, or satiety, reacts 

 on the organism more completely perhaps, whilst the 

 animal is young, but developmental changes are 

 induced at each moult of skin ; and beyond and inter- 

 mediate to these stated periods, modified growth and 

 change of form are efi"ected. Thus, in the moultings of 

 Lepidoptera, sometliing more is accomplished than the 

 formation of a larger skin for the insect. Important 

 changes take place, which are interestingly shown by 

 the casting of the internal mucous membranes of the 

 tracheae. 



The fact is significant that some beetles, like Meloae, 

 are born hexapod, and afterwards become apterous 

 maggots in the cells of bees, an economy discovered 

 some years ago by Newport. This has been considered 

 by some as a proof that the caterpillar form is adaptional 

 and not strictly developmental. 



Whilst in the embryo of the Crustacean the limbs 

 come into view before the segmentation, in Insecta 

 the segments precede the formation of the legs. Fritz 

 Miiller, and Packard also, thought it probable that 

 perfect insects appeared before larvse or pupge, and 

 that the vermiform type is more recent. They con- 

 sidered the primitive form was nearly allied to our 

 grasshoppers (Orthoptera). Relative to this observa- 



