The transport of prey by ants

Abstract

I. Myrmica rubra L. and Formica lugubris Zett. treated insect prey in one of two ways; either they licked and bit it without moving it or they siezed it and transported it to the nest. 2. The transport of prey to the nest was by carrying for light prey, and by dragging for heavy prey. 3. Ants dragging prey orientated visually, but the orientation was complicated in certain respects by the possession of prey. 4. When transport was difficult or impossible, for instance because the prey caught on a rough surface, ants swung out of their orientated line of march. This usually freed the prey, either by increasing the force on it through leverage or by sliding it sideways to clear the obstacle. 5. Single workers of M. rubra were able to exert forces of about 100 mg-wt against a spring, single workers of F. lugubris about 300mg-wt and pairs of F. lugubris workers from 400 to 600 mg-wt. The mean power developed by single M. rubra workers was 5.84 ergs/sec., of single F. lugubris workers 24.2 ergs/sec. and of pairs of F. lugubris workers 61.2 ergs/sec. Pairs of F. lugubris workers thus had an advantage over single ants of the same species in this respect. 6. The contribution which each of a pair of F. lugubris workers made to the resultant force they exerted against a spring was found. When the resultant force was small it was possible for the individual efforts of the two ants to be so applied that only a small proportion of them was contributed to the resultant force, and the rest was wasted by opposition between the two ants. When the resultant force was large more efficient application of effort occurred. Large forces and high efficiency were found only when the ants had been pulling together for 8-10 minutes. 7. In both species pairs of ants often failed to move prey because they pulled antagonistically. 8. When groups of F. lugubris moved prey in natural conditions the speed of movement of the prey was at first slow and often stopped as the number of ants attending it rose. After six or more minutes the speed of transport rose to a higher level. This was due to an increased orderliness which showed itself as a straightening-out of the line of transport. The spatial arrangement of ants in these groups was at no time random. Arrangements in which ants held opposite ends of the prey were significantly less common during the phase of straightened-out movement than at other times. 9. It is suggested that the changes in the efficiency with which forces were generated and with which prey was moved (referred to in paragraphs 6 and 8 of this summary) were brought about by the changes in position which ants made in response to difficulty of transport (paragraph 4).