ATTRACTIVE AND REPULSIVE ODORS 



405 



glands are found, both offensive and agreeable to their enemies and 

 friends. Some of these are external integumentary organs and others 

 are internal glands derived from the integument. We shall treat of 

 several of the prominent types here. The great variety and number of 

 these organs is astonishing. There are literally thousands of insects 

 that possess the structures in many positions on the body. In some they 

 are temporary larval structures. In most cases, as far as is known, 

 they are either integumentary invaginations or they are accessory anal 

 or oral glands. 



A fair type of an integumentary gland producing an ill-smelling fluid 

 is to be found in the earwigs or Forficulidae. The opening of these glands, 

 which are found at points on the latero-dorsal surface of the posterior 

 part of the abdomen, gives grounds for thinking that it is an integumen- 

 tary invagination. 



Dissection shows that the gland is shaped somewhat like a retort, or 

 is "pear-shaped." The narrow, funnel-shaped neck opens on the ex- 

 terior by a very tiny ringed opening in the chitin. The innermost layer 

 is a thin chitinous membrane that is striated irregularly and which gives 

 off plate-like and tube-like processes proximally into the underlying 

 (outer) cellular layer. This layer is composed of cells which are directly 

 continuous with the hypodermal cells of the outer integument, as also 

 the inner chitinous layer is continuous with the cuticle of the body. 



The next point is to see how the gland-cells are placed and how the 

 secretion is freed. The cellular cover- 

 ing is composed of two kinds of cells 

 (Fig. 367). The smaller are of no par- 

 ticular interest, and have small oval 

 nuclei and transparent bodies whose 

 boundaries are hard to see. The plate- 

 like processes of chitin extend into the 

 cytoplasm in places. The second sort 

 of cells are very large and of great 

 specialization and complexity. The nu- 

 cleus is extremely eccentric and flat- 

 tened somewhat. It is surrounded by 

 a thin, clear space. The larger part of 

 the cell body is occupied by a clear cyto- 

 plasm which contains and probably also 

 secretes the ill-smelling fluid. The fluid 

 is drained from the cell by one of the 

 chitinous tube-shaped invaginations of 

 the inner cuticle. This fine capillary lies with its end coiled up in the 

 clear area of the gland-cell. 



paec.r. 



FIG. 367. Two secreting cells from 

 the stink gland of the earwig, sec.r., 

 secretion region of the cytoplasm; 

 /., parts of tubule by which the secre- 

 tion is withdrawn from the cell. 

 (After VOSSELER.) 



