io8 THE BIOLOGY OF INSECTS 



of insects by the sets of activities which we generally call 

 instinctive. 



It is well known that during the pupal stage which 

 intervenes between the larval and the adult condition in the 

 vast majority of insects, the creature remains, as a rule, 

 quiescent and does not feed. In such a resting condition 

 the insect needs protection, and its behaviour when nearing 

 the close of larval life is largely concerned with this coming 

 need. Before the moult or casting of the last larval 

 cuticle which reveals the pupa (see Chap. VII, p. 172) 

 the full-grown larva often spins a silken cocoon, as do the 

 silkworm and many other moth- caterpillars, the cocoon in 

 some species being strengthened with the larval hairs or 

 bristles, or with foreign substances such as chips of wood or 

 particles of soil. Such cocoons serve as shelters for the 

 pupae resting within them. Or the larva, if it does not make 

 a cocoon, often seeks shelter by burying itself in the ground 

 as many hawk-moth and owl-moth caterpillars do, or by 

 creeping beneath a loose piece of bark, like the small cater- 

 pillar of the codling-moth that has fed within a growing 

 apple on a tree. 



But if a pupa lies enclosed in a cocoon, the perfect 

 winged insect, when developed, has to make its way out of 

 the cocoon after undergoing the moult that sets it free from 

 the pupal cuticle. Often it is found that some provision for 

 this need also is made beforehand by the larva. The silk- 

 worm's cocoon is left comparatively thin and weak at the 

 head end where the moth will have to come out, so that in 

 this region it is readily weakened and partly dissolved by a 

 fluid which the moth, when developed, discharges from its 

 mouth. The same provision is found in the behaviour of 

 the remarkable caterpillar of the Puss Moth {Cerura vinula) 

 which forms a hard and dense cocoon, but leaves in front a 

 weak area which is readily acted on by the strong alkaline 

 fluid that the emerging moth discharges from its mouth, as 

 O. H. Latter (1895) has shown. 



Still more remarkable, perhaps, is the behaviour of 

 larvae which feed in some object or substance out of which 



