168 A BOOK OF INSECTS 



defensive evacuations are commonly associated with warn- 

 ing coloration. 



Many insects are protected by the stiff hairs and 

 spines which cover their bodies ; while the spines of some 

 caterpillars are hollow and filled with poison. They break 

 to pieces when the insect is handled, and give rise to a 

 stinging sensation like that caused by the hairs of the 

 nettle and other plants. It is said that these stinging 

 hairs defend their possessors from almost all birds except 

 cuckoos. 



In the sub-order Homoptera, numerous species find 

 shelter beneath wax-like substances derived from their 

 own bodies. A definite shell, or " scale," is often formed, 

 as in the case of the mussel scale insect (Mytilaspis 

 pomorum) ; or the secretion may be thread-like or downy, 

 as, for instance, in the woolly aphis or American blight 

 (Schizoneura lanigera) of our orchards, and the felted 

 beech coccus {Crypt ococcus fagi). Among the frog- 

 hoppers the young nymph exudes a copious frothy liquid 

 — the familiar "cuckoo spit" — in the midst of which it 

 lies hidden ; while the caterpillars of certain saw-flies — 

 known popularly as " slug - worms " — secrete a dark- 

 coloured slime which completely envelops their bodies, 

 and renders them obnoxious to their foes. The larvse 

 of tortoise beetles (Cassida) actually shelter under their 

 own excrement, which accumulates as a kind of flattened 

 scale above the insect's back. 



Relatively few insects are endowed with defensive 

 weapons — if we except their mouth-parts, the primary 

 office of which is to procure food. But in certain families 

 of the Hymenoptera, and in these only, we find that re- 

 markable structure, the poison sting. It is an exclusively 

 feminine organ, being a modification of the egg-laying 

 apparatus, or ovipositor, and in many instances still retains 



