AQUATIC MIDGES AND SOME RELATED INSECTS. 27 



Chironomids labor under greater handicaps than other gnawing larvse in not having 

 strong legs provided with claws for holding them in position and in not having mandibles 

 that are opposable for cutting the tissue. Instead they have to depend upon indirect 

 methods, of applying pressure to their mouth parts, and the utilization of their labial 

 plates in conjunction with their mandibles for cutting the tissue. 



THE BURROW. 



The method of penetrating the epidermis in the beginning of the burrow is the 

 same for the leaves of Brasenia schreberi as for Casialia odorata, but from this point on 

 it differs markedly, due to differences in the texture of the plant. That chironomids 

 show adaptability in their feeding habits is well shown by the differences in their bur- 

 rows in B. schreberi and C. odorata, resulting from a difference in the thickness and 

 texture of the leaves of these two plants. 



We will take up first the nature of the burrow made on the leaves of Brasenia, 

 since they are thinner, more easily penetrated, and where equally available more seri- 

 ously attacked. This shows an evident selective power on the part of the larva. The 

 writer is, however, aware that the softness of texture may be the deciding factor. When 

 the larva has penetrated the epidermis of a leaf, it is able to bring pressure to bear more 

 directly on the labial plate. The mandibles are hooked under the edge, and they, 

 together with the pressure derived from the anterior prolegs, readily force the labial 

 plate down through the epidermis. Then the larva moves a little to one side and repeats 

 the operation. In this way the epidermis and parenchymatous tissue are removed from 

 under the silken arch. Then the larva commences in the mid line and makes a cut as 

 explained above from the center of the burrow to the outside edge. The larva during 

 this operation is inverted, with its head turned backward. It next assumes an upright 

 position, grasps the strip near its free end between its mandibles and labial plate, and 

 pulls the strip backward, raising it upward at the same time. Then by bending its 

 body to one side in its burrow it gets under the loose end and scrapes it clean of the 

 green parenchymatous cells that adhere to it and fastens it in place against the silken 

 arch (fig. 23). It next rakes this exposed area free of all the parenchymatous cells 

 down to the lower epidermis. This removal of tissue usually results in the admission 

 of water, probably through the mucous gland, as no openings are visible; at any rate 

 the burrow becomes flooded and capillarity keeps it wet both inside and out. 



The larva continues to cut slabs and to extend its burrow as long as it cares for 

 food. These slabs are twisted backward and fastened in an upright position and their 

 tips bound together with silk secreted by the salivary glands. The bottom as well as 

 the sides and top are lined with silk. In the leaves of Brasenia the bottom and sides of 

 the burrow have a very thin layer of silk which is closely applied to the surrounding 

 tissue. When castings are to be extruded, the larva turns about in its burrow and the 

 partially digested material is fastened to the arched top of the burrow by silk in such a 

 way as to serve as sort of a porch. It is held extended by silk threads which are fas- 

 tened out on the surface of the leaf. This porch or canopy serves a threefold purpose, 

 being a shelter from the sun, a means of retaining a film of water over the area that is 

 being excavated, and an entanglement in which the free end of the slab of epidermis 

 becomes lodged so as to be held up while the tissue is being removed from its underside. 



