from the two glands unite in a median thick-walled sac known as the silk press (Pr), which opens to the exterior through the spinneret. Two small accessory glands, which look like bunches of grapes and which are sometimes called the glands of Filippi (Fig. ?55 A, B, C, G/F), open into the silk ducts near their front ends. The relation of the silk ducts and the silk press to the spinneret is seen in the side view of the terminal parts of the labium and the left maxilla, given at B of Figure ?55- The silk press (Pr) is apparently an organ for regulating the flow of the liquid silk material into the spinneret. It
Flç. 156. The alimentary canal of the tent cater- pillar A, before feeding. B, after feeding. Cr, crop; Int, in- testine; Mal, Malpighian tubules; OE, oesophagus; Rect, rectum; l?ent, ventri- culus
has been supposed, too, that it gives form and thickness to the thread, but the liquid material has still to pass through the rigid tube of the spinneret. The cut end of the press, given at E of Figure ?55, shows the crescent form of the cavity (Lum) in cross- section, and the thickening in its roof (Rph), called the raphe. Mus- cles (Mcls) inserted on the raphe and on the sides of the press serve to enlarge the cavity of the press by lifting the infolded roof. The four sets of these muscles in the tent caterpillar are shown at Ç. The dilation of the press sucks the liquid silk into the cavity through the ducts from the reservoirs, and when the muscles relax, the elastic roof springs back and exerts a pressure on the silk material, which forces the latter through the tube of the spinneret. The continuous passageway from the ducts through
[ ?88 ]
THE
