AQUATIC MIDGES AND SOME RELATED INSECTS. 7 



with a mesad projecting process, which loosely articulates which the central chitinous 

 structure (figs, i and 2). These arms are provided with muscles and are capable of a 

 wide variety of movements. 



The lateral arms, while figured for a considerable number of species, do not seem to 

 have been treated at all from a functional standpoint. From the author's experience it 

 seems possible that the small size of the head and the constant activity of the larva have 

 served to vitiate many attempts in this direction. It is easy to see from the study of a 

 large number of dead larvse that the arms are to be found in a variety of positions, the 

 most frequently observed position being that found when the labrum is drawn in between 

 the maxillae. When the labrum is in this position, the arms project posteriorly down into 

 the pharynx, just above the surface of the hypopharynx. When the labrum is raised 

 somewhat, they are seen to lie just above the labium. When the labium is elevated, as in 

 the normal feeding, the ends of the arms are farther forward. 



Several times while examining the labium of living larvae the author has observed 

 what he considers the normal movements of these appendages. They are moved forward 

 and toward each other when the labrum is elevated, so that their setigerous anterior mar- 

 gins (fig. 2, a) scrape the chitinous claws (figs. 1 and 2, e) attached within the horseshoe 

 area, removing any food material that they may have collected. They are then swung 

 backward in close proximity to each other as the labrum is pressed down. From these 

 occasional observations, together with the structure of associated parts of the pharynx, 

 it seems reasonable to conclude that the lateral arms have an important function, as they 

 convey the food down the alimentary tract to such a level that the circular muscles of the 

 esophagus can act upon it in the swallowing process. They would, therefore, appear to 

 supplement the mandibles and maxilla, which may have lost something of their primitive 

 functions as an adaptation to their present manner of life. 



The mandibles have been figured by a large number of authors, especially from the 

 systematic standpoint. The author has tried to show in detail the method of articula- 

 tion of the mandibles with the head because of the restricted movements of these append- 

 ages resulting from their method of attachment. The anterior median margins of the 

 epicranial plate (fig. 5) carry on their inner surfaces special internal chitinous processes 

 (figs. 4 and 5, i) upon which the mandibles articulate. These processes alone would give 

 the mandibles a considerable freedom of movement. This movement, however, is some- 

 what restricted by the process q (figs. 3 and 6) and the plate st (figs. 4, 1 1, and 12). Their 

 chief movements are consequently confined to plains approximately a.t right angles to 

 each other. In this motion they oppose the labium rather than each other. The 

 complexity of the adductor muscles, however, enables the mandibles to oppose each 

 other when elevated. The external process of the mandible (fig. 3, q), which projects 

 beyond the point of articulation out over the thickened margin of the epicranial plate 

 (fig. 12, />;), adds considerable firmness and rigidity. The function of the mandibles is 

 of especial interest, because they, next to the labium, limit the range of adaptability of 

 the chironomids. This is especially emphasized in the discussion of the adaptability of 

 Chironomus brasenice and C. lobijerus. 



The maxilla (fig. 11) has been the object of considerable speculation especially as 



regards its homologies. Mundy (1909) figures vibrissa?, which he considers as replacing 



the striated structures shown in figures 11 and 12, c. This structure Goetghebuer 



(191 1) considers a part of the labium. The attachment of the movable parts of the 



80285°— 22 2 



