736 



THE NATIONAL GEOGRAPHIC MAGAZINE 



Another subfamily, the DoryHnse, em- 

 TDracing the wonderful driver ants of 

 Africa and the legionary ants of the 

 American tropics, are also highly carni- 

 vorous, but nevertheless succeed in form- 

 ing immense colonies, often of hundreds 

 of thousands of individuals. This they 

 accomplish by relinquishing the sedentary 

 habits so characteristic of the great ma- 

 jority of ants. They keep moving in 

 long files through the jungles, capturing 

 or killing all the insects they encounter, 

 and even overrunning dwellings, and. in 

 their search for cockroaches and other 

 vermin, driving out the human inhabit- 

 ants. From time to time these strange 

 ants bivouac for the night or for a few 

 days in some hole in the ground, or under 

 a tree, but soon continue their predatory 

 march. Evidently they are able to re- 

 main carnivorous, and at the same time 

 to develop large colonies, only because 

 they are nomadic and can thus draw 

 their food supply from a large area. 



The Ponerinae and Dorylinse ants have 

 thus adopted the only modes of life 

 which will permit a union of predatism 

 and sociability. 



Their colonies must either remain small 

 and rare, or, if populous, must keep mov- 

 ing from place to place. As each of 

 these conditions has serious disadvan- 

 tages, we find that the majority of ants 

 have preferred to become more and more 

 vegetarian, though, like man, usually 

 without completely abandoning their car- 

 nivorous appetites. 



A Favorite; :pood 



One of the earliest departures from an 

 exclusively animal diet is seen among the 

 ants which attend plant-lice, scale in- 

 sects, and leaf-hoppers and feed on their 

 saccharine excrement. This excrement 

 is, of course, merely plant sap slightly 

 altered in its chemical constitution by 

 passing through the digestive tract of 

 the insects, and containing much water, 

 some sugar, and a little nitrogenous 

 matter. 



Many ants are so inordinately fond of 

 this food that they not only acquire an 

 intimate acquaintance with the habits of 

 the adult plant-lice and scale insects, but 

 actually collect and store their eggs in 



the nests during winter in order that they 

 may during the ensuing spring distribute 

 the hatching young over the roots or 

 foliage of the plants. This is a well-de- 

 veloped habit among the species of Lasius 

 throughout temperate North America and 

 Eurasia (see pictures, pages 743, 744). 



Other plant juices, such as the nectar 

 of flowers and the similar liquid secreted 

 by the glands on the petioles of leaves 

 and by green galls, are also assiduously 

 collected by ants. There is, moreover, a 

 singular tendency for some ants which 

 are engaged in this collection of nectar 

 and plant-lice excrement to become 

 "honey-ants" through an extraordinary 

 exaggeration of the instinct to store 

 liquid food in the crop or social stomach. 



LIVING store;house;s for food 



Certain individuals, the "repletes," of 

 the colony refrain from leaving the nest 

 and foraging for food and become con- 

 verted into flagons by distending the crop 

 to such enormous dimensions that the 

 abdomen looks like a transparent bead 

 (see pictures, pages 747, 748). In this 

 condition they hang by their claws from 

 the roof of the nest chamber and thence- 

 forth spend all their lives receiving 

 liquid food from the tongues of the for- 

 a^ ig ants, storing it in their crops, and 

 regurgitating it to hungry individuals 

 when the liquid food supply outside the 

 nest becomes inadequate. This is, of 

 course, apt to be the case periodically 

 in dry regions, so that we find the true 

 honey-ants only in deserts like those of 

 the southwestern states, northern Mex- 

 ico, South Africa, and central Australia. 



In such localities also a further adap- 

 tation to vegetarianism may be frequently 

 observed in many species of ants which 

 take to harvesting and eating the seeds 

 of the small herbaceous plants. This 

 harvesting habit is evidently a last resort 

 in regions where insect food is very 

 scarce or confined to a brief season. 



There are several dozen species of 

 harvesting ants in North America alone, 

 the most conspicuous being those of the 

 genus Pogonomyrmex, the species of 

 wdiich range all the way from Montana to 

 the Argentine through the dry western 

 portions of two continents. It w^as for- 



