[6] Hist. Animal. l. iv. c. 19.
[7] The insection that distinguishes these parts, the abdomen especially, is most visible in the majority of the Hymenoptera and Diptera orders; next in some Coleoptera, as the Lamellicorn tribes, &c. and the Lepidoptera. Latreille is of opinion, that the two last segments of the thorax in some insects are represented by the first of the abdomen, and that the upper half segment of this part in Coleoptera also represents the same. Latr. De quelques Appendices, &c. Annales Générales des Sciences Physiques. A Bruxelles, vi. livrais, xviii. 14. In fact, in the Lepidoptera, when the abdomen is separated from the trunk, this segment usually remains attached to the latter. In the Myriapods, the trunk is to be distinguished from the abdomen only by its bearing the three first pair of legs.
[8] There is no general rule without exceptions, and no character is so universal as to be distinctly exhibited by every member of a class or other natural group. Thus, in the majority of the mites (Acarus L.) the body is marked by no segments, and the only articulation or incision is in the legs, palpi, &c. But as the exception does not make void the rule, so neither does the extenuation or absence of some primary character at its points of junction with others, in some individuals, annihilate the class or group.
[9] Hist. Nat. l. xi. c. 1.
[10] Animalcula polypoda, spiraculis lateralibus respirantia, cute ossea cataphracta; antennis mobilibus sensoriis instruuntur. Syst. Nat. ed. 12. i. 533.
[11] Quoted by Mr. Wm. MacLeay in his very remarkable and learned work Horæ Entomologicæ, in which he inclines to the same opinion. 383.
[12] Treviranus (Ueber den innern Bau der Arachniden, &c. 22.) always calls the palpi of spiders "Fülhörner." In Scorpio he regards them as palpi (Palpen).
[13] N. Dict. d'Hist. Nat. xvi. 181.
[14] Treviranus, ut supra, 48. For the nervous system of scorpions, see t. i. f. 13; and for that of spiders, t. v. f. 45.