170 A BOOK OF INSECTS 



sac serves as a reservoir whence formic acid is ejected 

 from an orifice at the tip of the abdomen. To inflict a 

 bodily injury these ants must first bite their enemy, and 

 then squirt poison into the wound ; but when thoroughly 

 alarmed they spray their acid into the air, which soon 

 becomes charged to suffocation with the penetrating 

 fumes. In this way the nests are defended against sudden 

 assault. 



The manner in which social insects make war upon 

 their enemies is very remarkable. Each member of the 

 clan seems animated by an unquenchable spirit of patriot- 

 ism, and concerted movements are carried out with the 

 utmost precision. The temper of the community varies 

 with the species. Wasps are undoubtedly more irascible 

 than bees, and many ants are characterised by a calcu- 

 lating aggressiveness which is especially marked in their 

 slave-hunting propensities. But in general terms it may 

 be said that all social insects are long-suffering, slow to 

 pick a quarrel, with little vindictiveness in their composi- 

 tion. Nevertheless, when once the security of their state 

 has been threatened, they declare war to the knife, as 

 those who have incautiously disturbed a wasps' nest know 

 to their cost. Many species depute sentinels to stand at 

 the threshold of the nest, and thus keep chance pillagers 

 and parasites at bay. 



The defensive value of an insect's physical attributes 

 — its armour-like skin, its powers of running, leaping, or 

 flying, and its extraordinary vitality — is too obvious to 

 call for emphasis ; but it is interesting to notice that these 

 endowments are bestowed in exact proportion to the needs 

 of each species. There is nothing incongruous in Nature's 

 scheme. Larvae which feed in concealment, or are the 

 object of tender solicitude on the part of adult indivi- 

 duals of their kind, are soft-skinned and defenceless ; while 



