198 AN INTRODUCTION TO ENTOMOLOGY 



I. The Development of the Legs of Nymphs and of Naiads 



In insects with a gradual metamorphosis and also in those with an 

 incomplete metamorphosis the nymph .or naiad when it emerges from 

 the eggshell has well-developed legs, which resemble quite closely 

 those of the adult. The changes that take place in the form of the 

 legs during the postembyronic development are comparatively slight ; 

 there may be changes in the relative sizes of the different parts ; and 

 in some cases there is an increase in the number of the segments of the 

 tarsus ; but the changes are not sufficiently great to require a descrip- 

 tion of them here. 



2. The Development of the Legs in Insects with a Complete Metamor- 

 phosis 



It is a characteristic of most larvae that the development of their 

 legs is retarded to a greater or less extent. This retardation is least 

 in campodeiform larvae, more marked in cruciform larvae, and reaches 

 its extreme in vermiform larvae. 



The development of the legs of insects with campodeiform larvae, 



Among the larvae classed as campodeiform the legs are more or less 

 like those of the adults of the same species ; there may be differences 

 in the proportions of the different segments of the leg, in the number 

 of the tarsal segments, and in the number and form of the tarsal claws ; 

 but these differences are not of a nature to warrant a discussion of 

 them here. These larvae lead an active life, like that of nymphs, 

 and consequently the form of legs has not been greatly modified from 

 the paurometabolous type. 



The development of the legs of insects with cruciform larvae. In 



caterpillars and other cruciform larvae the thoracic legs are short and 

 fitted for creeping ; this mode of locomotion being best suited to their 

 mode of life, either in burrows or clinging to foliage. This form of leg 

 is evidently an acquired one being, like the internal development of 

 wings, the result of those adaptive changes that fit these larvae to lead 

 a very different life from that of the adults. 



In the case of caterpillars the thoracic legs are short, they taper 

 greatly, and each consists of only three segments. It has been com- 

 monly believed and often stated that the three segments of the larval 

 leg correspond to the terminal portion of the adult leg; but studies of 

 the* development of the legs of adults have shown that the divisions 

 of the larval leg have no relation to the five divisions of the adult leg. 



