70 O. E. PLATH. 



pp. 25-31). However, this explanation is rejected by von Buttel- 

 Reepen (1903), who suggests that, on the contrary, bumblebees, 

 being guided solely by pleasant stimuli in such cases, probably 

 incubate their brood because they profit by the warmth which 

 is given off by the larvae. Several well-founded objections to von 

 Buttel-Reepen's (pp. 38-39) theory have been published by 

 Wagner (1907). Among other things, Wagner (pp. 93-95) has 

 pointed out that bumblebees incubate objects from which they 

 cannot possibly get warmth, a statement which is corroborated 

 by my own observations. Thus, for example, I have frequently 

 seen queens and workers of various species incubate the outer 

 rim of tin cans which were used in artificial breeding experiments, 

 even when the thermometer was below the optimum temperature 

 for bumblebees. While external stimuli undoubtedly play an 

 important part in this interesting habit of bumblebees, broodiness 

 probably also depends upon a certain physiological condition. 

 Only in this way, it seems to me, can one explain the fact that it 

 sometimes takes several weeks until a confined Bremus queen 

 becomes broody, and that, after becoming so, she readily incu- 

 bates various objects, such as tin cans or the floor and sides of 

 her nest-box, in some cases even days or weeks before she lays 

 her first eggs. 



VIII. RED CLOVER AND POLLEN. 



In his monograph on bumblebees referred to at the beginning 

 of this paper, Schmiedeknecht (p. 328) claims that bumblebees 

 cannot obtain pollen from clover. This statement likewise is 

 incorrect. The corbiculce of bumblebees working on red clover 

 almost always contain dark-brown pollen, and by a half hour's 

 observation one can easily convince oneself that this pollen is 

 obtained from the clover. 



IX. METHOD OF DEPOSITING POLLEN. 



Another point on which there is a difference of opinion, is the 

 question as to how a bumblebee deposits her load of pollen. 

 Coville (1890), whose observations agree with those of Sladen 

 (1912), describes this activity as follows: "If pollen-laden, the 

 bee balanced herself, with her middle and cephalic pairs of legs, 

 on the edge of a pollen-pot, head outward, spread her wings, and 

 then scraped the pollen-masses from her corbiculce by rubbing the 



