Allocation of liquid food to larvae via trophallaxis in colonies of the fire ant, Solenopsis invicta

In social insects, the size, caste and reproductive capacity of adults is determined in part by nutrition during larval development. Among ants, workers bring food to immobile larvae, giving workers potential control over larval nutrition, and making social feeding a potential mechanism of individual and colony ontogeny. During each regurgitation (trophallaxis), workers feed larvae the same, small increment of liquid food, regardless of larval attributes and conditions. Therefore, differences in the total volume of food ingested by larval resulted from differences in the rates of trophallaxis to them, not from differences in the durations of trophallaxis. The rates of worker-to-larva trophallaxis (feedings/h) were examined to investigate the mechanisms by which liquid food is allocated to larvae by workers. The rate of trophallaxis increased with larval food deprivation. The magnitude of this increase depended upon the larva’s size. When larvae were food-deprived, larger larvae were fed at significantly higher rates than were smaller larvae (13 feedings for each pl of larval volume). Once larvae of all sizes were satiated, workers fed them at similarly low rates. Regardless of size, larvae required about 8 h of feedings to reach satiation; that is, small larvae did not become sated any sooner than did medium or large larvae. Rates of trophallaxis were independent of: (1) the size or hunger of adjacent larvae, (2) rates, of larval encounter by workers (larvae were encountered hundreds of times per h but were fed only tens of times); (3) larval location on the brood pile (top or bottom); and (4) larval body orientation (mouthparts up or down). These results provide the first quantitative evidence that an individual larval hunger cue directs the feeding of each larva, and that the strength of this cue, and therefore the feeding rate, varies with larval size and hunger. c 1995 The Association for the Study of Animal Behaviour The reproductive success of a wide variety of insects is determined to a large degree by nutrition during the larval period. Larval nutrition can affect adult fecundity directly through increased reserves for egg production or indirectly through increased female body size. In social Hymenoptera, larval nutrition takes on special importance because it can influence the caste of the resulting adult (Wheeler 1994). Social feeding, in which adults feed immobile larvae, is a potential mechanism for regulating adult caste by means of nutritional switches that determine which of a small number of discrete developmental options a female larvae will follow (Wheeler 1986, 1990, 1994). The outcomes of these developmental options are the various female castes making up insect colonies: queens and workers Correspondence: D. L. Cassill, Unit One, Department of Biology, Florida State University, Tallahassee, FL 32306-3050, U.S.A. (email: [email protected]). of single or multiple sizes. Thus, larval feeding may be a central, regulatory process in colony ontogeny. Social feeding differs markedly among eusocial Hymenoptera. Wasp and bee larvae are reared individually in cells that are provisioned when food supplies become low (Pendral & Plowright 1981; Huang & Otis 1991; Hunt 1991). By contrast, ant larvae are reared communally in brood chambers, creating potential difficulties in assessing individual larval needs. The ability of ant workers to rear larvae successfully under such potentially confusing conditions has not been widely appreciated or investigated (Hiilldobler & Wilson 1990). Recent studies have revealed fairly complex organization of ant brood by developmental stages within the communal brood chamber. Workers sort brood, maintaining eggs and firstinstar larvae separate from older larval instars within the same brood chamber, while keeping 0003-3472/95/090801+ 13 $12.00/O a, 1995 The Association for the Study of Animal Behaviour