PROTECTIVE RESEMBLANCE 143 



examples are the caddis-worms, which construct portable 

 dwellings of small stones, leaves, twigs, fragments of rushes, 

 or the shells of aquatic molluscs. In these they pass the 

 larval and pupal stages of their existence, vacating them 

 only when they quit the water to assume the adult state. 

 Similarly, the larva? of some clothes moths make cases, 

 using fragments of the cloth upon which they feed ; while 

 in the family Psychidce the caterpillars employ leaves, 

 grasses, or pieces of heather for the same purpose. 



Many caterpillars spin leaves together in order to 

 provide a shelter for the critical pupal period, and this 

 habit probably gave rise to the cocoon-making instinct 

 which is highly developed in certain families. Normally 

 the cocoon consists of silk, and it has been proved that 

 at least some species — e.g. the emperor moth (Saturnia 

 jjavonia) — can adjust the colours of their cocoons, making 

 them either light or dark in accordance with the environ- 

 ment. Other species work foreign objects in their cocoon, 

 such as leaves, moss, or pieces of twig, thereby strengthen- 

 ing it and rendering it less conspicuous. The cocoons of 

 the "puss' : and "kitten" moths (genus Cerwa) are 

 formed from fragments of bark mixed with silk acted 

 on by formic acid. They are extremely difficult to 

 discover, as they are exactly like natural flaws or excres- 

 cences of the bark itself. It should be noted that the silk 

 glands of caterpillars, caddis-worms and Hymenopterous 

 larvae are thought to be homologous with the true salivary 

 glands. They open in the region of the mouth ; whereas 

 those of certain Neuropterous, Coleopterous and Dipterous 

 larvae (also spiders) have their place at the posterior end 

 of the abdomen. Silk is a fluid secretion which hardens 

 rapidly when exposed to the air. 



