34 BULLETIN OF THE BUREAU OF FISHERIES. 



toothed margin. There are two pairs of these pectinate hairs, which, together with 

 the other spines and processes, make quite a formidable and closely set array of scraping 

 implements. The labium is, on the whole, not very different from the one figured for 

 Chironomus brasenice except in the number and relative lengths of the teeth. In Metrioc- 

 nemus knabi the teeth of the labium are of nearly uniform length. 



The maxillae are quite remarkably different. The inner mesad projecting portion 

 has a number of long close set spines which lie in the same plane as the labium and 

 doubtless are of great assistance in supplementing it and the hypopharynx in then- 

 scraping action. The basal portion labeled for Chironomus brascnice is in Metriocnemus 

 knabi narrower and less firmly united to the epicranial plate. The fan-shaped structure 

 marked c (figs. 1 1 and 12) is entirely lacking in this species. 



The hypopharynx is long and well supplied with short, blunt processes. Its margin 

 lacks the chitinized plates shown for Chironomus brascnice and is obviously specialized 

 as a delicately sensitive scraping structure. Its general appearance is that of a soft 

 and somewhat flexible tonguelike structure covered with processes that are doubtless 

 tactile in function. The epipharynx is provided with the usual pair of lateral arms which 

 are here similar but less strongly developed than is the case with Chironomus brasenus. 



FEEDING HABITS. 



Knab's remark that the larvae burrow among the fragments of decayed insects and 

 evidently obtain their food from this source is true as far as it goes. It leaves one in 

 doubt, however, as to the actual food of the larvae. Several times the author saw larvae 

 with broad chitinous bands around their bodies, evidently segments of insects' legs. 

 Other observations have shown the larvae with considerable parts of their bodies extend- 

 ing into these narrow insect appendages where the larvae were apparently feeding. 

 This would lead one to think that the larvae feed upon the decaying tissues found there. 

 This is doubtless true, for they are quite adaptive in their habits; but from other observa- 

 tions it would seem that they were, perhaps, even here feeding indirectlv on the insect 

 structures by devouring the large numbers of bacteria that in turn break down the insect 

 tissues. This conclusion has been reached after considerable experience with these 

 larvae under artificial conditions. 



Larvae were removed from the pitcher plants and placed in petri dishes, together 

 with the insect debris in which they were living. They were found to be perfectly well 

 fitted to live in this manner. It was also found that they could live on beef broth, 

 smoked beef, and decaying plant material. It is obvious, therefore, that insects are not 

 the only source of food for these larvae. 



The question at once arises, Why is this species found so universally in the pitcher 

 plant and nowhere else if it can live upon so wide a range of food ? The answer is obvi- 

 ously given in the adaptation of this larva to a particular kind of food. In this adapta- 

 tion the mouth parts are doubtless most fundamental. As explained above, they are 

 fitted with a number of combs, spines, and fingerlike processes. The structure and 

 length of the hypopharynx also indicate that it, too, is used as a delicate scraping organ, 

 which is of prime importance in assembling the scattered bacteria. In this connection 

 the presence of well-developed silk glands in close association with the hypopharynx 

 suggests the possible function of their secretion in assembling the bacteria upon which 

 the larva largely subsists. 



