42 concentrate near the nucleoli, and vacuoles appear in the cells; the 



branching of the nucleus becomes more complex (Figure 49). The outer part 

 of the tube is lined with cells with simple nuclei and a thick intima. 



The intima retains its elasticity in the press but is denser and thicker, 

 forming an oblong, ridgelike process with an apical tubercle on the wall of 

 the tube; the dorsal ridge projects into the tube of the press; the lumen of 

 the press becomes sickle-shaped in cross section (Figure 46); unlike in 

 caterpillars of Lepidoptera, only two pairs of muscles are attached to the 

 outer walls of the press. The contraction of the muscles widens the lumen 

 of the press, so that it becomes filled with the secretion; the elasticity of 

 the walls closes the lumen, forming the silk; the silk fibers have a slightly 

 flattened cross section (Figure 47) and are divided into two parts by a deep 

 groove. The secretion of the silk glands is a viscous liquid hardening in 

 water to brownish silk. If larvae are fixed, the secretion of the glands be- 

 comes hard; when a fixed larva is dissected, the hardened secretion can be 

 extracted in the form of two brown, strong curved rods. The larvae produce 

 a large quantity of silk, so that a larva of Hydro psyche can build its 

 very complicated chamber in two hours. 



The pedal glands and Gilson's glands have an accessory function in the 

 building activity of the larvae. The pedal glands (Figure 50) are especially 

 well developed in the Polycentropodidae (Liibben, 1907: Figure 2; 

 Brickenstein, 1955:13, Abb. 10); the main function of the secretion of these 

 glands is adhesive. 



(44) 



FIGURES 51-53. Gilson's glands: 



51 - simple Gilson gland and the prosternal horn of Platyphylax designatus (A), Limno- 

 philus extricatus (B); distal part of the horn and opening of Gilson's gland of L. flavicor- 

 nis (C); 52 - secretory cells of Gilson's gland of Glyph ota el i us pellucid us; 53 - branched 

 Gilson's glands of the prothorax and mesothorax of Phryganea sp. (51, A - after Gilson; 51, B and 

 53 - after Henseval; 51, C - after Brocher; 52 - after Zarechnaya). 



pg - prothoracic ganglion, mesg - mesothoracic ganglion, subg - subesophageal ganglion. 



39 



