lOO PRESIDENTIAL ADDRESS SECTION D. 



B. The Physogastric Condition of Tennitophiles. — In certain 

 of the more highly modified termitophiles there occurs the re- 

 markable condition of the body known as physogastric. The 

 swollen state, which is generally limited to the abdomen, is 

 mostly met with among certain Staphylinid termitophiles, but 

 it must be remembered that it is not confined to them. It is 

 developed in a pronounced degree in the highly specialised ter- 

 mitophilous dipteron Termitoxenia, and it is also found in various 

 coleopterous and dipterous larvae which live in the nests of 

 termites. It is thus clear that the physogastric condition is not 

 due to any special peculiarity in the nature of the Staphylinid 

 beetles, since a similar response to the termite environment arises 

 in such different insects as a beetle and a fly. Physogastrism 

 is not a marked feature among myrmecophiles, although Dr. 

 Brauns informs me that there is some evidence that the little- 

 known larvge of the myrmecophile Paussidcc tend to be more 

 or less physogastric. 



A much swollen abdomen occurs in the young and in all the 

 castes of termites, and it has been suggested that the condition 

 is associated with the fact that the food of termites consists so 

 largely of carbohydrates. Since the termitophiles either feed 

 on the fluids of the young or are fed by the workers, it might be 

 imagined that the food received would induce in them a similar 

 physogastric state. Such a purely physiological cause will not, 

 however, wholly explain the phenomenon in the termitophiles, 

 since if this was the chief cause we should expect all closely 

 allied termitophiles to be more or less physogastric, and this 

 is not the case. In addition to any direct physiological efifect 

 of the nature of the food, there are apparently a number of 

 other equally or more important causes which combine to 

 favour physogastrism in certain termitophiles, and these are 

 now summarised : 



(i) The physogastric condition of the termitophile may 

 have been assisted by natural selection so that the intruder might 

 resemble more closely the termites themselves. We have 

 seen that the condition is normal among termites, and a termito- 

 phile, if of the same kind of size as the young or adu4t termite, 

 would be less distinguishable by the tactile sense, and would be 

 more readily acceptable to the community if the shape of the 

 body mimiced that of the termites. For example, the physo- 

 gastric Staphylinid Paracorotoca is of the same colour as the 

 termites, and with its swollen abdomen it is not easily detected 

 when mixed with a crowd of workers ; also, the supposed larva 

 resembles both in colour and in general shape the young ter- 

 mites. 



(2) The mimicing eft'ect, however, is less obvious when 

 the abdomen of the termitophile is altogether larger than that 

 of the termites. Such is the case in the large termitophilous 

 larvae of the Carabid beetles Orthogonius and Glyptus, but it 



