354 INSECTA. 



of the digestive organs with what we have already described in the 

 Butterfly (fig. 160), to appreciate the amazing dissimilarity : it would 

 be difficult, indeed, to imagine, did not anatomy convince us of the fact, 

 that the digestive apparatus of the imago, with its slender oesophagus, 

 dilated crop, short sacculated stomach, long and convoluted small in- 

 testine, and capacious colon, was derived from a gradual modification of 

 such viscera as those we have just been considering. The salivary 

 glands of the Caterpillar (fig. 178, q, r) are large cylindrical caeca; and 

 their ducts (p) pour into the mouth an abundance of saliva proportioned 

 to the coarse nature of the materials used as food. 



(917.) The sides of the body are traversed by the wide longitudinal 

 tracheae (a b c), that communicate on the one hand with the lateral 

 spiracles, and on the other give off at regular intervals the air-tubes 

 (d,eee e), which ramify most minutely over all the viscera, and convey 

 the atmospheric air throughout the entire system. 



(918.) Besides the above organs, there are other viscera, which, 

 although of considerable importance to the Caterpillar, would be utterly 

 useless to the imago, and consequently are more or less completely want- 

 ing in the mature state. 



(919.) The whole body of the larva is filled with a peculiar fatty tissue 

 (fig. 178, ///), called by entomologists the rete, epiploon, or fat-mass. 

 This material, found in great abundance in mature and well-fed larvae, 

 consists of an oily or greasy substance enveloped in a most delicate 

 cellulosity, and seems to correspond to the fat of higher animals, like 

 which it is indubitably a product of digestion, and a repository of super- 

 abundant nourishment, stored up, no doubt, for the sustenance of the 

 animal during its helpless condition in the dormant or pupa state 

 serving, like the fat of hibernating quadrupeds, for food during the con- 

 finement of the imago. 



(920,) But the most remarkable peculiarity of the larvae under con- 

 sideration is the presence of an apparatus employed for producing a 

 tenacious thread of extreme delicacy, appropriated by different species 

 to various purposes. In many cases (fig. 148), it is made subservient to 

 locomotion ; and by its assistance, as by a rope, the larva can suspend 

 itself from any object, or let itself down from one branch to another in 

 search of food. The most important uses, however, to which this thread 

 is applied are connected with the concealment and protection of the 

 quiescent and defenceless pupa; either furnishing the means of sus- 

 pending the chrysalis in a place of safety* (fig. 179), or, as is the case 



* For a most amusing account of the manner in which some chrysalides manage, 

 without any external limbs, to suspend themselves by the tail in a position of security, 

 the reader is referred to Kirby and Spence, vol. iii. p. 207. Fig. 179 (in the next page) 

 illustrates the different steps attending the process. The larva (A), having spun some 

 loose silk, and fixed it upon the under side of a leaf or other suitable object, suspends 

 itself therefrom by its hind-legs. The skin of the caterpillar then gradually splits down 

 the back (B, c), arid is slowly pushed upwards towards the tail of the chrysalis. The 



