AQUATIC MIDGES AND SOME RELATED INSECTS. 13 



METHOD OF SPINNING SILK. 



The method by which Chironomus lobiferus larvae spin or spread out the silk used in 

 the construction of their burrows and in the formation of the little conical nets mentioned 

 above is very simple. The anterior pair of prolegs is the chief implement employed 

 and so far as can be observed the only part of the body used for this purpose. The 

 structure of these appendages takes on a new significance when function is suggested, 

 and we at once notice the difference in structure between the anterior and posterior 

 prolegs. 



The chitinous claws of the posterior pair are widened at their base (fig. 33), are 

 few in number, and are arranged around the front and lateral margins of the prolegs 

 (fig. 15). The muscles of the prolegs are so arranged as to set these hooks into the 

 silken lining of the burrow, and thus hold the larva firmly in place. The hooks point 

 outward and are so attached that by the contraction of the muscles of the proleg they 

 are all brought close together in the center. When extended, the hooks all move 

 outward in different directions, with the result that the prolegs are hooked fast to the 

 silk lining of the burrow. Their function is preeminently that of an attachment, and 

 it is to this specialization of the posterior appendages that the anterior prolegs owe their 

 greater freedom of movement. 



The anterior prolegs are often mistaken for a part of the head because of their 

 position just posterior to the chitinized portion of the head proper. They commonly 

 appear as a mass of bristles radiating in all directions. From the side they appear as 

 one, because they are always moved together and are so completely covered by rela- 

 tively long spines that it is hard to see how they are attached. A sagittal view shows 

 them to be made up of two rounded lobes separated by a narrow depression. The 

 spines are graded in length from mere tubercles in front to long narrow hooked and 

 barbed spines in the centre and again decreasing in size on the posterior surface. Here 

 the short spines have rather wide bases and the tips are deeply serrate and somewhat 

 hooked (fig. 34). The spines are obviously arranged in rows which diverge somewhat 

 from the mid line laterally (fig. 35). The spines located near the centre of the prolegs 

 are the best developed and are probably the most used in silk spinning. They are 

 curved backward and hooked at their tips. Near the end there are a number of barbs 

 on both the anterior and posterior edges. They are flattened laterally and are capable 

 of being condensed into a very compact mass by the contraction of the muscles of the 

 proleg. The hooks at the end of the spines point backward, and all the long spines 

 are hooked except a few of the very outer spines, which seem to be slimmer and more 

 hairlike. 



The actual process of silk spinning is much more easily studied by observing the 

 construction of the conical net mentioned above than in any other way. It is constructed 

 out free from other substances and is consumed and replaced every 10 minutes night 

 and day until the activities of the larva are slowed down by the approach of the pupal 

 stage. The larva begins the spinning by extending its body well forward and making 

 several fairly rapid passes with its anterior prolegs in various radial directions. These 

 movements place the silk strands that form the attachment for the apex of the net. 

 Then, withdrawing its body somewhat and attaching the silk to the place where these 

 radiating strands fuse with each other, the larva retracts its body, drawing out a ribbon 



