SCENT ORGANS IN TRICHOPTERA. 469 
Beneath this sac is another sac which opens by a longitudinal 
slit along the inner margin of the palpus beneath the opening of 
the first sac. At first I thought this sac was only an artefact, 
imagining that the membrane lining the hard chitinous convex 
surface of the palpi had come away. But sections showed the 
existence of a distinct cellular lining to the inside of this chitin, 
and there can be no doubt that the sac in fact exists. 
Text-figure 6. 
Sericostoma personatum, 6. 
Palpus of the first maxilla, seen from the inside with most of the silky hairs 
removed (diagrammatic). X 39. 
ST. Stipes. L. Lip. LB. Lobe. FL. Flap. 8S). Entrance to sac 1. 
So. Entrance to sac 2. S. Scent hairs. ZT. Tip of palpus. 
A study of transverse sections of the palpus affords us the 
following histological information. 
Text-fig. 7 is through the line AB in text-fig. 6. It shows 
the two sacs and the septum between them, and the layer of 
large, deeply stained, glandular cells at the base of the silky 
hairs. 
The hypodermis beneath the outer, convex cuticle of the palpus 
is composed of a double layer of flat squamous cells. The 
septum between the two sacs is formed by the innermost cell- 
31* 
