66 INTRODUCTION. 



tions is in the number of feet, a circumstance which 

 necessarily gives rise to different modes of progres- 

 sion, and occasions striking differences in habit and 

 appearance. Before alluding, however, to the num- 

 ber and arrangement of the feet, it will be proper 

 to mention what peculiarities are observable in the 

 structure of these members. In many instances 

 there is no fleshy plate at the extremity of the pro- 

 legs capable of being expanded and contracted to 

 serve the purposes of a foot, the leg being simply a 

 conical fleshy prominence, having the extremity 

 surrounded by a complete coronet of hooks. Ex- 

 amples frequently occur in which the prolegs have 

 very much the appearance of a wooden leg, the upper 

 part being thick, succeeded by a slender cylindrical 

 piece which terminates in a circular expansion sur- 

 rounded with crotchets, and having a small nipple 

 in the centre which holds the place of a foot. 

 Although these small hooks are generally present, 

 this does not seem to be universally the case, for 

 the subcutaneous larvae of a small moth of the 

 Linnean genus Tinea, and a few others, are said to 

 be without them. The true, or pectoral legs, are 

 always six in number, and nearly uniform in figure ; 

 the most remarkable among the few exceptions to 

 this, is to be found in the caterpillar of the Lobster- 

 moth, which has the two posterior pairs greatly 

 elongated and terminating in a kind of claw. The 

 amount of abdominal legs, however, is very variable 

 in different groups, and in the anomalous caterpillars 

 of two small brownish-yellow moths (Heterogenea 



