490 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



APPROACHING COOPERATION 



In the termites' nest it is obvious that adjustment must have oc- 

 curred ; in other cases it may not be so obvious. Wliere mutual adjust- 

 ment of behavior cannot be seen we must be careful to avoid the 

 conclusion that any advantage gained from being in a group is the 

 result of cooperation. The larvae of the white pine weevil feed 

 under the bark of twdgs, eating their v^ay down towards the base of 

 the shoot. The number of grubs in a shoot is always just enough to 

 eat away the plant tissues all round the twig. If there are too many 

 larvae, some are crowded out and cannot feed, while if there are too 

 few, resin flows sideways from uneaten tissues into the damaged area 

 and kills the larvae. Clearly the larvae gain an advantage from 

 group feeding, but as there is no adjustment of their behavior to suit 

 that of their fellows (no alteration of the rate of feeding or of the 

 width of cut, for instance) the advantage is not the result of coopera- 

 tion. 



A closer approach to cooperation is to be found in young larvae of 

 the jack pine sawfl3^ These caterpillars feed in groups on the foliage 

 of Pinus hanhsiuna (pi. 1) ; caterpillars feeding singly are very rare. 

 The aggregation is not imposed on the caterpillars by the way the 

 female laid her eggs. Groups form on needles where no eggs were 

 laid, and will reform if the caterpillars are artificially spread out over 

 the foliage. However, regrouping does not occur if the caterpillars 

 are spread out on a sheet of paper. This behavior is related to a defi- 

 nite situation — feeding — and it is this which provides the key stimuli. 



A. W. Ghent has shown that groups form around feeding cater- 

 pillars which have succeeded in penetrating the hard cuticle of the 

 leaf. The situation provides the necessary stimuli for grouping — the 

 smell of damaged foliage and of a resinous secretion produced by 

 feeding larvae. Since the small first-stage larvae have difficulty in 

 biting through the cuticle, breaks in it are important for their survival. 

 Young caterpillars make full use of any presumably lucky break in 

 the cuticle by extending the cut edge. Therefore caterpillars feeding 

 in a group are better able to feed and correspondingly more survive to 

 their first molt. 



As these groups are formed by adjustment of the behavior of some 

 larvae to use the success of others, the caterpillars can be said to 

 cooperate in exploiting these situations. They respond to the evidence 

 of success — the smell of damaged leaves. The cut in the leaf is the 

 only link between members of the group, for sometimes larvae ap- 

 proach the opposite side of the cut to the larva that started it, and they 

 move away from each other as they feed. 



