NO. 2 RECOGNITION AMONG INSECTS McINDOO 57 



evaginated and the tuft of hair is expanded, this organ greatly 

 resembles a cylindrical fan whose contents are turned inside out to 

 form the circular part of the fan. Freiling asserts that scent hairs 

 are more common than scent scales in abdominal scent-producing 

 organs. A few of these hairs are innervated and vacuoles seem to 

 be always absent in the large gland cells at the bases of the hairs, 

 but a secreting tubule is invariably present. The scent hairs may have 

 parallel ridges with myriads of fine pores in the chitin between the 

 ridges, or they may have many stubby, thornlike projections, each 

 of which is pierced by a small canal through which the secretion 

 passes to the exterior. The scent hairs are filled with a matrix 

 substance. 



lO. ANAL GLANDS AS SCENT-PRODUCING ORGANS 

 (a) ANAL GLANDS OF COCKROACHES 



Bordas (1901) describes a voluminous organ in the posterior end 

 of the abdomens of males belonging to Periplaneta orientalis and 

 P. americana. This organ is a slender sac with a series of dicho- 

 tomously branched tubes running into its anterior end. The posterior, 

 or narrower end of the sac opens to the exterior by an oval aperture 

 in a chitinous projection under the penis. The gland itself lies in 

 the ventral portion of the abdominal cavity at the right, and all of 

 its tubes are surrounded their full length by a layer of unicellular 

 glands. A cross section of one of these tubes shows three layers in 

 its wall as follows: (i) The outer layer is a thin membrane; (2) 

 the middle one is a layer of trapezoidal gland cetls ; and (3) the inner 

 one is the chitinous lining of the tube. Each gland cell has a large 

 nucleus and a vesicle from which runs a filamentous, secreting tubule 

 to the chitinous canal, the collecting tube. This gland secretes a 

 volatile, strong and nauseating liquid, sometimes acrid and alliaceous, 

 recalling the odor of a mouse or that from old cheese in decomposi- 

 tion ; it secretes continuously but in time of danger its action is 

 accelerated. 



Harrison (1906) describes a supposedly new organ in Periplaneta 

 orientalis. This glandular organ lies on the ventral side in the 

 sixth abdominal segment and opens to the exterior between the sixth 

 and seventh sterna. From the external opening the organ extends 

 upward and forward as two distinct lobes. 



(b) ANAL GLANDS OF BEETLES 



Dufour (1811) first described the anal gland of the bombardier 

 beetle, Brachinns displosor. This gland is paired and one half of it 



