SCENT ORGANS IN TRICHOPTERA. 471 
layer which runs up to encircle a number of muscle-fibres. This 
septum is stiffened by a comparatively thick chitinous intima 
(text-fig. 7, CL), which runs in from the entrance to cover about 
half of the outside wall of the inner sac. 
In the sections, the “lip” (LL) has been accentuated owing to 
the sinking in of the tissues beneath it to form a hollow where 
the fluted edges of the sections of the dark hairs are seen. 
Under this lip the two squamous layers of cells are widely 
separated the one from the other and the space between them 
traversed by strands. ‘The cells are also larger. 
On working round to the inner surface of the palpus one finds 
the coneavity full of an immense thickness of hairs seen to be 
circular in cross-section and containing a central canal of small 
bore. The cuticle supporting these hairs is produced into elongate 
papille containing the alveoli in which the scent hairs are fixed. 
Beneath the cuticle the hypodermis consists of a glandular 
epithelium of elongate cells, specialised formative cells called by 
Graber trichogens, in which the scent is secreted [(10) p. 188]. 
On account of the fact of the immense number of these cells and 
of the hairs which they support, it has not been easy, from an in- 
spection of sections of unfixed material, to say definitely whether 
there is a single trichogen cell to each hair or whether there are 
several (text-fig. 8). Usually in the scent organs of the Lepi- 
doptera there is one cell—one hair; but Bertkau (20), in the 
ease of the Noctuine genera Hadena and Dichronia, points out 
that there is not one giant cell to each of the hairs of the scent 
tuft (which are of enormous length), but several smaller cells 
belong to each hair. 
That these hairs may act as scent organs it is necessary for 
them to remain in connection with the living hypodermic tissue. 
Text-fig. 8 shows how a pore-canal or channel runs up to the 
base of each alveolus through the chitinous papilla, thus putting 
into communication the cutaneous appendage with the hypodermic 
trichogen cell. 
No opening pores were found either at the base of or at the 
tip of the hairs, and probably the scent secretion runs up the 
canal within the hairs by capillary attraction and becomes 
diffused by osmosis into the outer air. In the androconia and 
scent hairs of the Lepidoptera it is now generally held | Berlese 
(12) p. 533] that the secretion reaches the air by osmosis, as 
apertures in the integumentary appendages are no longer thought 
to exist. 
The cuticle of the inner surface is plicate and the hypodermic 
cells are much smaller. Strands of tissue run across between 
the hypodermis of the inner and outer layers, being very clearly 
seen where the two layers of cells diverge from each other in 
the “lip.” 
It will be remembered that Miiller (supra, p. 460, Section i) de- — 
scribes the male during mating as separating the maxillary palpi 
and spreading out the hair around the head * wie ein Heiligen- 
