510 MisceUaneous. 



peculiar organs of tliese Acarina, the nature of •which has not up 

 to the present been established. 



I. Porous area. — We know tliat in the females of the Ixodidoe 

 there are found on the dorsal surface towards the base of the 

 rostrum two finely punctured depressions called porous areas. 

 Authors have been content to point them out without indicating 

 their exact nature ; I think that it may be inferred from my 

 observations that these two pits rej)resent a sensory organ. 



In sections the porous area is seen to be composed of a series of 

 apertures or pores which pass right through the chitin, and, when 

 regarded as a whole, exactly recall the appearance of a sieve. 

 Beneath each aperture there lies an ovoid nerve-cell Avith a central 

 nucleus. The nerve- cells terminate on the dorsal side in short 

 prolongations in the shape of little rods, which penetrate into the 

 apertures of the porous area. By their other extremity they are 

 attached to a bundle of nerve-fibrils, which spreads itself out 

 widely in the form of a fan on the under side of the organ. In a 

 series of sections the nerve can be traced fairly easily as far as the 

 latero-anterior region of the brain, where it originates. 



By reason of its structure there seems no doubt that the porous 

 area represents a sense-organ that may be compared with the 

 lyriform organs and their analogues of the Arachnids. Nevertheless, 

 there is in this case a more marked specialization, since this organ 

 exists only in the females. 



II. Eye. — The eyes of the Ixodidae are of the simple type, that 

 is to say, they are composed of a crystalline lens, a vitreous body, 

 and retinal cells. The lens set in the chitin of the cephalothorax, 

 of which it is but a simple differentiation, is white, hyaline, and 

 strongly convex; in Hyalomma offine, Neumann, it is 150 jx thick 

 and has a diameter of 100 fj. It is not composed of concentric 

 lamelljc, but exhibits striai perpendicular to its surface ; these 

 stria? are numerous and accentuated by a black pigment contained 

 in the crystalline substance. Beneath the crystalline lens the 

 hypodermis is prolonged to form the vitreous body, which is com- 

 posed of low compressed cells. This body is bounded by a circular 

 zone of tall cells, corresponding to the irisated zone or tapete of the 

 Arachnids. The retinal portion comprises a small number of ovoid 

 nerve- cells of large size (30 ^i by 20 /n), with posterior nuclei, as in 

 the nocturnal eyes of the spiders and in those of the Opilionidae. 

 I have not found coloured pigment cither between the retinal cells 

 or upon the margin of the vitreous body, as we see it regularly in 

 the other Tracheata. 



We find, then, that the eyes of the Ixodida3 diverge in certain 

 respects from those of the Arachnids, and are characterized : (1) by 

 the great thickness of the lens and the strong curvature of the 

 crystalline body ; (2) by the presence of black pigment in the 

 crystalline body ; (3) by the absence of pigment between the retinal 

 cells and in the irisated zone ; (4) by the great size of the nerve 

 cells. 



