iv.J METAMORPHOSES OF INSECTS. 77 



known as alternation of generations, may have ori- 

 ginated. 



Summing up, then, the preceding argument, we find 

 among insects various modes of development ; from 

 simple growth on the one hand, to well-marked 

 instances of the so-called alternation of generation on 

 the other. In the wingless species of Orthoptera 

 there is little external difference, excepting in size, 

 between the young larva and the perfect insect The 

 growth is gradual, and there is nothing which would, 

 in ordinary language, be called a metamorphosis. In 

 the majority of Orthoptera, though the presence of 

 wings produces a marked difference between the 

 larva and the imago, the habits are nearly the same 

 throughout life, and consequently the action of ex- 

 ternal circumstances affects the larva in the same 

 manner as it does the perfect insect. 



This is not the case with the Neuroptera. The 

 larvae do not live under the same conditions as the 

 perfect insects: external forces accordingly affect them 

 in a different manner ; and we have seen that they 

 pass through some changes which bear no reference 

 to the form of the perfect insect : these changes, how- 

 ever, are for the most part very gradual. The cater- 

 pillars of Lepidoptera have even more extensive 

 modifications to undergo ; the mouth of the larva, 

 for instance, being remarkably unlike that of the 

 perfect insect. A change in this organ, however, 

 could hardly take place while the insect was grow- 

 ing fast, and consequently feeding voraciously ; nor, 

 ever if the change could be thus effected, would the 

 mouth, in its intermediate stages, be in any way fitted 



