340 ARACHNIDES. 



that they can only be detached by force, and by tearing' out the portion that 

 adheres to it. They lay a prodigious quantity of eggs, and sometimes in- 

 crease to such an enormous extent on the Ox and Horse, that they perish 

 from the exhaustion. Their tarsi are terminated by two hooks inserted in 

 a palette, or united at base on a common pedicle. 



The ancients designated these Arachnides by the term Ridnus. They 

 are our Ticks. 



CLASS III. 

 INSECTA. 



Insects, which form the third class of articulated animals provided 

 with articulated legs, have, besides, a dorsal vessel analogous to the 

 vestige of a heart, but totally destitute of any branch for the circu- 

 lation. They respire by means of two principal tracheae, extending, 

 parallel to each other, throughout the whole length of the body, hav- 

 ing centres, at intervals, from which proceed numerous branches, 

 corresponding to external openings or stigmata, which admit air. 

 They all have two antennae and a distinct head. The nervous sys- 

 tem of most Insects — the Hexapoda— is generally composed of a 

 brain formed of two opposing ganglions, united at base, giving off 

 eight pairs of nerves and two single ones, and of twelve ganglions, 

 all inferior. By counting the eight pairs of the brain, and the ten 

 spinal bridles, which may also be considered as so many pairs of 

 nerves, we shall have in all forty-five pairs, exclusive of two solitary 

 nerves, or from twelve to fourteen more than are found in the hu- 

 man subject. In our general remarks on points common to the 

 three classes of articulated animals provided with articulated feet, 

 we mentioned the various opinions of physiologists with respect to 

 the seat of the sense of hearing and of smell. We will merely add, 

 in regard to the former, that certain little nervous frontal ganglions 

 seem to confirm the opinion of those who, like Scarpa, place it in 

 the origin of the antennoe. I have detected two small orifices near 

 the eyes of certain Lepidoptera, which, perhaps, are auditory canals. 

 If, in several Insects, particularly those furnished with filiform, or 



