224 Biology in America 



etc., but occasionally they take to preying upon each other, 

 oftentimes with results fatal alike to cannibal and to prey. 

 Powers records his observations of some cannibals as follows : 

 'It was curious that in every instance where two or more 

 of the cannibals were placed at close quarters, even though 

 other larrce were present, the result was the destruction of 

 one or both of the cannibals, while the others frequently 

 remained unharmed. This result is not due to the natural 

 enmity of competitors or to a wise foresight with regard to 

 a limited food supply, but purely to the strongly modified 

 reactions of the cannibals themselves. While an ordinary 

 larva instinctively avoids close contact with another, and 

 beats the most precipitate retreat at the merest touch of 

 cannibalistic jaws, the possessors of these weapons them- 

 selves are apparently wholly divested of this innate fear. 

 Unless Decidedly hungry they lie sluggishly at the bottom, 

 either ignoring the chance contacts of other specimens, or 

 savagely nabbing the intruder. The violence and instan- 

 taneousness of their occasional movements contrast strongly 

 with their sluggish inactivity between whiles. Even com- 

 plete satiety does not usually check their savage attacks, pro- 

 vided that the proper stimulus is offered; the prey is then 

 seized and held some time or half swallowed, to be then "as 

 quickly rejected by a sudden jerk much like the one by which 

 it was seized. Thus it is that cannibals in close proximity 

 almost invariably prove each other's undoing, the swallower 

 frequently succumbing as well as the swallowed. Even when 

 taken in the reservoir, not a few of the broad-heads were 

 sadly bitten or abraded, some having been, it would seem, 

 nearly swallowed before meeting with the resistance, no doubt, 

 of some friend who had just gone before." 1 



Not only may ordinary individuals be induced to assume 

 the cannibalistic habit, with resultant change of shape, but 

 cannibals may occasionally be reformed and made to resume 

 the ordinary shape. Obviously the larval form is very plastic 

 and readily amenable to the influence of habit. The greatly 

 exaggerated head with its wide gape is clearly the result of 

 swallowing large prey, while the emaciation of other parts 

 of the body, especially gills and legs, Powers ascribes to the 

 "energy absorbed in the heroic efforts of ingestion" and 

 the nourishment required for the excessive development of 

 head, digestive tract and body length. 



The change of plumage in birds in the spring and fall 

 months is a commonplace, but none the less wonderful phe- 

 nomenon, a satisfactory explanation for which has not yet 



1 "Studies from the Zoological Laboratory/' University of Nebraska, 

 Vol. IV, pp. 57-58. 



