DEVELOPMENT 



143 



Eruciform Larvae. The prevalent type of larva among holometab- 

 olous insects is the cruciform (Fig. 213, -/), illustrated by a caterpillar 

 or a maggot. Here the body is cylindrical and often fleshy; the integ- 

 ument weak; the legs, antennae, cerci, and mouth parts reduced, often 

 to disappearance; the habits sedentary and the sense organs corre- 

 spondingly reduced. These characteristics are interpreted as being 

 results of partial or entire disuse, the amount of reduction being pro- 

 portional to the degree of inactivity. Extreme reduction is seen in the 

 maggots of parasitic and such other Diptera as, securing their food with 

 almost no exertion, are simple in form, thin-skinned, legless, with only 

 a mere vestige of a head and with sensory powers of but the simplest 

 kind. 



Transitional Forms. The cruciform is clearly derived from the 



FIG. 214. Mantispa. A, larva at hatching thysanuriform; B, same larva just before 

 first molt now becoming cruciform, C, imago, the wings omitted; D, winged imago, 

 slightly enlarged. A and B after BRAUER; C and D after EMERTON, from Packard's Text- 

 Book of Entomology, by permission of the Macmillan Co. 



thysanuriform type, as Brauer and Packard have shown, the continuity 

 between the two types being established by means of a complete series 

 of intermediate stages. The beginning of the cruciform type is found 

 in Neuroptera, where the campodeoid sialid larva assumes a quiescent 

 pupal condition. The key to the origin of the complete metamorpho- 

 sis, involving the erucif orm condition, Packard finds in the neuropterous 

 genus Mantispa (Fig. 214), the first larva of which is truly campodei- 

 form and active. Beginning a sedentary life, however, in the egg-sac of 

 a spider, it loses the use of its legs and the antennae become partly 

 aborted, before the first molt. In Packard's words, "Owing to this 

 change of habits and surroundings from those of its active ancestors, it 



