188 PTILOTA. 



time to time fresh supplies of water, from which it extracts 

 the air, as represented in a subsequent page. 



Hut in other aquatic larvae, as in the Ephemera, the sides 

 of the body are furnished with elongated flattened plates, 

 through which a slender air tube meanders, and which com- 

 municates with the longitudinal air tubes above mentioned 

 (fig. 32, p. 200). Other variations occur in the larva: of the 

 gnats (Culicida:), and midges (Chironomns), which we have 

 already described. But the most curious variation which 

 occurs in this respect is found in the very common larvae of 

 the Helophilus pendulus, which has been termed the rat-tailed 

 grub, from the peculiar formation of the extremity of tin- 

 body, which is very slender and elongated, inclosing a second 

 still more slender air tube, which is capable of being pro- 

 truded, so as to be pushed to twelve times the entire length 

 of the body. As the insect lives in mud, this structure is 

 eminently serviceable in enabling it to obtain a due supply 

 of air. 



There are some considerations resulting from the varia- 

 tions in the form of the larvae of insects which ought not to 

 be passed over without notice, inasmuch as the subject is one 

 of great interest, hitherto but little cultivated, and one of 

 much importance as regards the classification of the nnuulose 

 sub-kingdom. It has been observed by some recent physio- 

 logists, that the immature state of insects typified the perfect 

 forms of those particular grades which may be supposed to 

 have preceded the winged type in the progress of creation 

 from the lowest to the highest and most perfect forms. M it h- 

 out arriving at this theory, Mr. MacLeay, in his Ilorae Ento- 

 mologies:, and Messrs. Kirby and Spence, have given a series 

 of analogies exhibited by the larva: of insects with other au- 

 nulose groups in the perfect state; the former contending, 

 that it is only 1\ the assistance of Mich analogies that tin- real 

 mode of distribution (by which every variation shall have its 



