8 BULLETIN OP THE BUREAU OF FISHERIES. 



maxilla is especially interesting in this connection. The structures below p (fig. n) 

 are attached to the chitinous plate st (fig. n) along its outer margin. The parts 

 marked / and g (fig. n) are attached to the part beneath the letter p (fig. u) and are 

 capable of being folded over it. They articulate at w (fig. n) and swing inward. In 

 attempting to homologize these structures the plate si (figs, n and 12) is considered as 

 representing the stipes, which is fused to the anterior margin of the epicranial plate p. 

 Evidently p (fig. 11) represents the palpus and the structures below it (/>) the palpifer; 

 / and g represent the combined lacinia and galea and c the two cardos, one beneath the 

 other. 



The movements of the maxillae are restricted by their attachment to a fixed plate 

 outside the plane of movement of the mandibles. It seems probable that their function 

 has been largely taken over by the labrum and especially the lateral arms. The anterior 

 part (fig. 4, g and /) is capable of a considerable movement in a lateromedial direction 

 and although rather thin is doubtless an important factor in the concentration of the food 

 particles. This part of the maxilla, as well as the palpifer, carries a number of sense 

 papillae which doubtless have more or less well-developed taste cells, as it is easy to see 

 that the larvae have very acute taste organs in this part of the head. It therefore seems 

 probable that as the function of the maxilla has decreased the maxilla itself has become 

 very much modified. 



The labium in the family Chironomida? is very important in the determination of 

 the larva and consequently is a familiar structure in the systematic literature. This 

 structure is developed as a thickened plate with an anterior toothed margin. It is so 

 closely fused with the lower surface of the epicranial plate that in many species it seems 

 to be only a modification of the anterior border of this part of the head. This is especially 

 noticeable in those species which show a suture between these plates in the labial region. 

 This structufe is, however, capable of being removed as a separate plate, and more 

 complete study will doubtless show a similar arrangement throughout the subfamily. 

 Its function is that of a scraping and cutting edge, and it is next to the mandibles in its 

 importance in governing the range of adaptability of the species. 



The hypopharynx (figs. 7 and 8) is furnished with chitinous plates th and a variety 

 of spines and setae. This anterior portion is separated from the posterior by a cavity 2 

 (fig. 7), which is continuous with the salivary ducts d (fig. 8). It is supported by a 

 chitinous ring / (fig. 8) . The posterior part is furnished with a large number of backward 

 pointing setae on- its dorsal surface and is supported by a chitinous skeleton shown in 

 figure 8, k and /. The arms (fig. 7, k) of the hypopharynx form a point of attachment 

 for the upper end of the pharynx and hold this part extended. 



The function of the hypopharynx is doubtless sensory to a large extent, as its r61e 

 of guarding the entrance to the alimentary tract and the exit of the salivary ducts 

 would naturally demand. It seems probable that the backward projecting setae (fig. 

 7, e) at the entrance of the pharynx may also serve to disentangle the food material 

 brought in by the lateral arms. The structure of the anterior and posterior borders of 

 the cavity in the hypopharynx through which the silk escapes is of special interest in 

 connection with the study of the silk structures spun both by this and other species of 

 Chironomidae, although the part which it plays is still uncertain. 



