6 BULLETIN OF THE BUREAU OF FISHERIES. 



STRUCTURE AND FUNCTION OF HEAD OF Chironomus brasenise WITH 

 REFERENCE TO FEEDING HABITS. 



The head of the chironomid larva, while so constructed as to be wonderfully adapted 

 for feeding upon a large variety of foods in diverse environmental conditions, nevertheless 

 shows a wide range of variations. These variations, while especially well marked in the 

 subf amilies, are also to be found among the different genera and to a lesser extent within 

 the genus. They have been taken advantage of by the systematists, who have figured 

 the structures that best lend themselves to their purposes. Miall and Hammond (1900) 

 and more recently Goetghebuer (191 1) have made more careful studies of these structures, 

 with special consideration of their morphology. The object in this discussion is therefore 

 to consider the special adaptation of the mouth parts, with particular reference to their 

 function in the feeding habits. As certain of these structures have already been treated 

 more fully than others, only those parts whose function appears to the author to be either 

 poorly or inadequately discussed elsewhere will be considered in great detail here. 



In this study the head parts of Chironomus brasenice n. sp. are figured, and an at- 

 tempt is made to point out the more conspicuous differences between this species and the 

 larger and better known species upon which Miall and Hammond (1900) worked. The 

 most noticeable feature about the head of the larva of C. brasenice is its great width rela- 

 tive to the length. The labrum is also unusually narrow and the head has a roughly 

 triangular outline (fig. 10). 



The labrum undergoes a remarkable amount of variation in minor details, such as 

 the presence or absence of a thin triangular labral comb, variously arranged pectinate 

 hairs, and paired lobular bodies. The structures located on the ventral surface of the 

 labrum are commonly assigned to the epipharynx and consist of a three to many toothed 

 epipharyngeal comb located on the anterior border, a thickened chitinized horseshoe- 

 shaped area just posterior to it, within which are attached a variable number of claws or 

 spines, and just outside of these spines a pair of peculiarly mandiblelike structures, known 

 as premandibles (Goetghebuer) or lateral arms (Johannsen) . The corresponding struc- 

 tures of C. brasenice are peculiar in being reduced in number, larger in size, and more 

 strongly chitinized as an adaptation to its leaf-eating habits. The function of the labrum 

 is that of a very complexed scraping organ, and the degree of specialization of its various 

 parts is usually found to be correlated with the nature of the food and its method of 

 collection. 



The labrum is rather less specialized in this species than in Chironomus cayugce and 

 the others included by Goetghebuer (191 1) in his Group I. The pectinate hairs are fewer 

 in number and simpler (figs. 1 and 2, s). The epipharyngeal comb (figs. 1 and 2, co) con- 

 sists of three large rounded teeth with smooth inner surfaces. The horseshoe-shaped 

 chitinous area (figs. 1 and 2, h) on the ventral surface of the epipharynx is in this species 

 much less horseshoe shaped than usual. It is here represented by two chitinous bars 

 which articulate in front with the thickened anterior border of the labrum and poste- 

 riorly with a median caudad projecting process (fig. 1) within this horseshoe area. There 

 are four pairs of chitinous hooks (figs. 1 and 2 , e) , rather blunt in outline in Chironomus 

 brasenice, but often very much specialized and developed as minutely serrate plates. Just 

 lateral to the posterior end of the horseshoe-shaped chitinous area are the " lateral arms" 

 of Johannsen or "premandibles" of Goetghebuer (figs. 1 and 2, a). These are provided 



