188 PTILOTA. 



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

 the air, as represented in a subsequent page. 



But in other aquatic' larvae, as in the EpheraercE, 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 variu.ions occ: v in the larvae of the 

 gnats [CidicidcB), and midges {Chironomus), which we have 

 already described. But the most cmious variation which 

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

 the Helophiluspendulus, which has been termed the rat-tailed 

 grub, from the peculiar formation of the extremity of the 

 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 cidtivated, and one of 

 much im})ortance as regards the classification of the annulose 

 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 tjqje in the progress of creation 

 from the lowest to the highest and most perfect forms. With- 

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

 mologicae, and Messrs. Kirby and Spence, have given a series 

 of analogies exhibited by the larvse of insects with other an- 

 nulose groups in the perfect state ; the former contending, 

 that it is only by the assistance of such analogies that the real 

 mode of thstribution (by which every variation shall have its 



