AQUATIC MIDGES AND SOME RELATED INSECTS. II 



abundance of openings near the broken end of stems, and that the penetration is more 

 easily accomplished in a small gallery, where the larva is able to brace its body against 

 a somewhat resistant parenchymous tissue, is obvious when the nature of the larval mouth 

 parts is understood. 



These structures have been fully discussed in connection with Chironomus brasenuB, 

 and it is only necessary to consider them very briefly here. The labium is used as a 

 cutting edge and is applied at an angle of about 45 ° to the surface. Pressure is brought 

 to bear upon it by the mandibles, which on a flat surface have to be widely extended 

 in order to bring their pointed tips into use. This pressure is therefore applied very 

 largely as a sidewise pull and has the effect of using the labium more or less like a scraper. 

 Hence, the strength of the larva and the toughness of its labial plate are important 

 factors limiting its attack on plant tissue. 



USES MADE OF SILK. 



The fact that the larva of Chironomus lobifcrus lives as it does in a burrow which 

 communicates with the exterior by two small openings, too small to allow the larva 

 within to extend its body, naturally makes one curious to know how it is able to obtain 

 food. The natural conclusion, of course, would be that it ate the plant tissue, but 

 this is not found to be the case when the stomach content of the larva is examined. 

 Willem (1908) observed this and stated that the stomach content was composed of 

 organic debris analogous to that which floats in the water — "desmids, diatoms, Pedi- 

 astrum, Clathrocystis, spicules of Spongilla, carapace of hydrachnids, rotifers, together 

 with grains of sand and sometimes the fragments of plant diaphragms." The author's 

 study of stomach contents fully corroborated the above observations, although at the 

 time the author was not acquainted with Willem's work. 



In the author's study of the behavior of the larvae a number of burrows were cut 

 from the stems with just enough tissue to prevent disturbing the silk lining. These 

 preparations were placed in Syracuse watch glasses and observed under a binocular 

 microscope. Considerable difficulty was encountered in seeing through the epidermis, 

 so it was cut away and replaced with a cover glass. The larva readily readjusted 

 themselves by making their burrows open at the ends of the section of tissue instead 

 of up through the epidermis. In this way the behavior could be watched much more 

 exactly, but it was not until one of the most characteristic performances, over an area 

 where the underlying tissue had been entirely removed, was observed that a clew to 

 the method by which the larvae obtain their food was discovered. Willem (1908) dis- 

 misses this subject by stating that the food is removed by adhering to the walls of the 

 burrow near the end at which it enters. The following statement, translated from the 

 same source, seems to refer to the movement that gave this clue: 



Sometimes the larva is fixed posteriorly retracting and elongating in the act of going and coming 

 rhythmically, its body playing the role of a piston for renewing the water in the tube. 



This movement, so well described by Willem, the author has been able to demon- 

 strate is concerned in the spinning of a thin conical net across the end of the burrow. 

 This net is used to strain the floating organisms out of the water which the larva forces 

 through it by the rhythmic undulatory motion of its body. In this process the larva 



