204 



may be said to enter on the pupa state " (of course immature 

 pupa state, must be understood) as soon as it has assumed the 

 flaxseed form and color is a mere matter of opinion, and is of 

 httle consequence so long as we understand what is the actual 

 form of the insect included in the flaxseed shell. It wears the 

 form of a larva, whether you call it one or not. So the C. salicis^ 

 C. rohinice and C. tritici, after the larva has ceased eating, has 

 become quiescent, and is waiting the development of the limbs 

 of the fly, before these actually become visible, — are all, during 

 this state, exactly in the condition of the Hessian Fly insect 

 after assuming the flaxseed form externally, — and during this 

 period they all retain the larval form. The mere name given to 

 the insect during this period will not change the facts. The 

 caterpillar of a butterfly, when it has come to its growth, ceases 

 to eat, seeks a suitable place and then suspends its body. It 

 remains hanging awhile, still wearing the form of a caterpillar, 

 and then becomes a pupa by casting off its caterpillar skin. 

 The condition of the insect between the active state of the 

 caterpillar and the pupa state is exactly analogous to the con- 

 dition of the Cecidomyians during the period above mentioned. 

 The difference between them is this, — the caterpillar casts off 

 its skin to take the form of a pupa ; the Cecidomyians do not 

 cast off their (apparent ?) larva skins to take the form of a pupa. 

 By the " form of a pupa " you will understand that I mean a 

 form different from that which the insect previously had, one in 

 which the members of the future fly are more or less distinctly 

 visible, beneath the skin that envelops them.^ This peculiarity 

 in the transformation of the Cecidomyians, described in my last 

 letter, I should not have credited if I had not seen it myself. It 

 was a suspicion that the transformation of these insects had 



1 The true or mature pupa is that condition of the insect in which the limbs, etc., of the 

 future fly are developed but still remain in an immature state, each separately inclosed 

 in a portion of what you call the scarf-skin, just as the fingers of the hand are in a glove. 

 'Phis kind of ii pupa is said to have " the limbs free " because they are not (as in the 

 Lcpidoptera) soldered to the breast. 



If 



