460 DIPTERA CHAP. 



that protrudes externally, as shown in Fig. 2 1 9, A, there is a longer 

 portion concealed, forming a, sort of handle, having muscles 

 attached to it. Some of these larvae have the power of executing 

 leaps, and he states that such larvae are provided on the terminal 

 segment with a pair of corneous papillae ; bending itself almost 

 into a circle, the larva hooks together the breast-bone and 

 the papillae, and when this connection is broken the spring 

 occurs. This faculty is only possessed by a few species, and it 

 is probable that in other cases the spatula is used as a means for 

 changing the position or as a perforator. Some of the larvae 

 possess false feet on certain of the segments. Williston says 

 they probably do not moult. In the pupal instar (Fig. 219, B), the 

 Cecidomyiid greatly resembles a minute Lepidopterous pupa. 

 The Hessian fly, Cecidomyia destructor, is' frequently extremely 

 injurious to crops of cereals, and in some parts of the world 

 commits serious depredation. The larva is lodged at the point 

 where a leaf enwraps the stem ; it produces a weakness of the 

 stem, which consequently bends. This Insect and C. tritici (the 

 larva of which attacks the flowers of wheat) pupate in a very 

 curious manner : they form little compact cases like flax-seeds ; 

 these have been supposed to be a form of pupa similar to what 

 occurs in the Blow-fly ; but there are important distinctions. The 

 larva, when about to uuderwo its change, exudes a substance from 

 its skin, and this makes the flax-seed ; the larval skin itself does 

 not form part of this curious kind of cocoon, for it may be found, 

 as well as the pupa, in the interior of the " flax-seed." Other 

 Cecidomyiids form cocoons of a more ordinary kind ; one species, 

 described by Perris as living on Pinus maritima, has the very 

 remarkable faculty of surrounding itself, by some means, with a 

 cocoon of resin. "Walsh describes the cocoon-forming process of 

 certain Cecidomyiids as one of exudation and inflation ; Williston 

 as somewhat of the nature of crystallisation. Some Cecidomyiids 

 are said to possess, in common with certain other Diptera, the 

 unusual number of five Malpighian tubes ; and G-iard says that 

 in the larva there is only a pair of these tubes, and that their 

 extremities are united so as to form a single tube, which is 

 twisted into an elegant double loop. 



Thirty years or more ago the Eussian naturalist, Wagner, made 

 the very remarkable discovery that the larva of a Cecidomyiid 

 produces young ; and it has since been found by Meinert and 



