THE ORGANIZATION OF INSECT SOCIETIES 



425 



nest-building activities for their captors. 

 Polyergus workers starve, even in the pres- 

 ence of abundant and suitable food, if de- 

 prived of their slaves. In observation nests, 

 one slave can keep ten slave-makers well 

 fed. The original colony of slave-makers 

 probably is founded by a queen that in- 

 vades a Formica nest of an appropriate 

 species, kills the queen, and takes her 

 place. When the Formica population is de- 

 pleted, the slave raids replenish the popu- 

 lation of slaves. 



SHELTER AMONG SOCIAL INSECTS 



The nests of the social insects stabilize 

 the physical environment and thus control 

 various ecologic factors at more optimal 

 values (pp. 428, 439, 672). Nests of prim- 

 itive social insects, are often merely 

 burrows in the soil or in dead wood. 

 Elaborate structures may be built of earth 

 particles glued with organic excretions and 

 secretions (termites, ants), or plant mate- 

 rials such as wood particles or chewed bark 

 (termites, ants, wasps), of excrement (ter- 

 mites), or secretions from special glands 

 (wax of bees). Certain nests may stabilize 

 temperature (Figs. 150, 151), humidity, or 

 gaseous exchange (Figs. 231-233). Some 

 termite nests have rain-shedding covers or 

 ridges (Figs. 154, 235), The nests of some 

 social insects may serve for the storage of 

 food or the cultivation of fungi. Most nests 

 probably protect their builders from preda- 

 tors. The social homeostasis attained 

 through nest construction often enables 

 these insects to inhabit otherwise unfavor- 

 able habitats. For example, the mound 

 nests of ants and termites in periodically 

 flooded grasslands or swampy regions en- 

 able the insects to maintain ecological posi- 

 tion (p. 671) in relation to water. Nest con- 

 struction and shelter tubes enable termites 

 to become abundant insects in desert re- 

 gions in spite of their susceptibility to 

 death from evaporation in dry air. 



The structuies built by social insects are 

 usually formed of dead or nonliving mate- 

 rial external to the insect. Some nests, how- 

 ever, are composed partly or wholly of 

 secretions. Rarely, as in the bivouacs of 

 army ants or the clusters of bees, the 

 bodies of the insects themselves may be 

 used for homeostatic control (p. 431; Fig. 

 152). In each of these cases, subtle and 



often highly complex social behavior and 

 cooperative eflFort are manifest. 



DEFENSE FROM PREDATORS 



In the more primitive social Hymenop- 

 tera, the worker caste defends the colony 

 from external attack. In the more highly 

 integrated insect societies of ants and ter- 

 mites, the defensive function is performed 

 primarily by the soldier caste. Primitive 

 termites (i.e., Kalotermitidae) lack adult 

 workers, and only soldiers constitute the 

 adult sterile caste. More specialized ter- 

 mites (i.e., Termitidae) have 3 to 16 per 

 cent soldiers in the adult sterile-caste popu- 

 lation. The percentage of soldiers gathering 

 at points of attack or disturbance may be 

 as high as 100 per cent (Fig. 149), 



Defensive adaptations of the soldier ant, 

 like that of the antecedent solitary hunting 

 wasp, is often linked with predatory ad- 

 justments. In some instances, however, it 

 is possible to separate the adaptations for 

 oflFense from those for defense. The soldiers 

 of the ant genera Colobopsis and Crypto- 

 cerus have phragmotic heads that have 

 convergently evolved as plugs for the en- 

 trance holes of the nests (p, 233, Fig, 63), 

 Each colony of Cryptoceriis occupying an 

 enlarged hollow twig contains only one or 

 two soldiers, whose only function seems to 

 be that of being "doorman" for the colony, 

 preventing the entrance of predators and 

 trespassers, and allowing the workers easy 

 entrance and exit. The soldiers of certain 

 genera of termites (Cryptotermes, Glypto- 

 termes, and their relatives) have evolved 

 phragmotic heads (also convergently) that 

 plug the internal burrows and thus protect 

 the colony from invasion. The mandibles of 

 these soldiers are somewhat reduced, com- 

 pared with those of their relatives whose 

 soldiers have no phragmotic adaptation. 



The minor soldier of the termite, Rhino- 

 termes, has reduced mandibles; the labrum 

 is prolonged into a slender grooved struc- 

 ture with a forked pubescent tip. A fluid 

 from the opening of the frontal gland in 

 the head is exuded, rolls down the groove 

 of the labrum and rests at the tip until it 

 evaporates, in this manner producing a re- 

 pellent and somewhat toxic gas. The major 

 soldier in the same colony has large biting 

 mandibles, and the frontal gland is much 

 smaller than in the minor soldier. 



