THE PROBLEM OF DEFENCE 107 



charge can be repeated several times in rapid succession, 

 though with diminishing force ; and as the volatile acid 

 is sufficiently corrosive to stain the human skin a rust-red 

 colour, one can imagine its demoralising effect upon the 

 pursuing enemy. The entomologist Westwood relates 

 that individuals of a large South American JBrachinus, 

 when seized, " immediately began to play off their artil- 

 lery, burning and staining the flesh to such a degree that 

 only a few specimens could be captured with the naked 

 hand, leaving a mark which remained for a considerable 

 time." 



Some pine-feeding saw-fly larva? eject a resinous liquid 

 from their mouths when irritated. They are gregarious ; 

 and it is a remarkable fact, observed by many naturalists, 

 that if one larva is touched, the whole colony instantly 

 responds by a concerted twitching movement — every 

 individual contributing its quota of the strong-smelling 

 shower. This habit probably helps to defend the larva? 

 against the attacks of ichneumons. The caterpillars of 

 swallow-tail butterflies are endowed with a retractile, 

 Y-shaped tentacle behind the head. Normally it lies 

 within the body ; but it is shot out when the insect is 

 alarmed, and diffuses a penetrating odour, which, in the 

 case of our British species, agrees with that of the food- 

 plant — the hog's fennel or milk parsley. Many other 

 insects emit offensive or noxious secretions, either from 

 the mouth, or from glands situated on different parts of 

 the body ; while the blood itself sometimes serves as a 

 repellent fluid, issuing from a pore at the extremity of 

 the femur. This is the case with the ladybirds (Cocci- 

 nettidce) and the oil beetles (Meloidce). In the latter 

 family, the blood contains cantharidine, an extremely 

 caustic substance, which is an almost perfect protection 

 against birds, reptiles, and predaceous insects. Such 



