[36] There is some reason for thinking, though the octopod and myriapod insects breathe by tracheæ, that there is no small difference in the distribution of these organs. The Trachean Arachnida have only a pair of spiracles, from which the tracheæ must radiate, if I may so apply the term, in order to convey the necessary supply of air to every part of the body. Scutigera, as far as I can discover, has only a single series of dorsal spiracles (see Plate [XXIX.] Fig. 20)—an unusual situation for them: in these also, to attain the above end, each trachea must also radiate, so as to supply each part of the segment it is in. Those of Iulus, according to the observations of Savi (Osservaz. per servire alla Storia di una Specie de Iulus, &c. 15—), consist of bundles of parallel tracheæ. Perhaps these circumstances would warrant the considering of these Arachnida and the Myriapoda as primary classes? The genus Galeodes is said to breathe by gills similar to those of the Araneidæ, which structure, probably, carries with it a system of circulation, and exhibits a third type in the Arachnida, with four palpi, six legs, and a distinct thorax. This genus, then, is the corresponding point in the Arachnida to the Hexapod Aptera, as the Scorpions are to the Cheliferidæ or Pseudo-Scorpions, and the Araneidæ to the other Octopods; and these analogies furnish a strong proof, that the Tracheans belong rather to Insecta than Arachnida. Comp. N. Dict. d'Hist. Nat. xxvi. 445; and Description de six Arachnid. nouv. &c. par Leon Dufour, 16.
[37] Mr. MacLeay observes with regard to the Tardigrade, described by Spallanzani and Dutrochet, that "it proves that an animal may exist without antennæ or distinct annular segments to the body, but having two eyes and six articulate legs." (Hor. Entomolog. 350—.) Many Acari prove the same thing. De Geer, vii. t. vii. f. 14.
[38] De Geer, vii. t. iii. f. 8.
[39] Hor. Entomolog. 351.
[40] De Geer, Ibid. 571, 583. t. xxxvi. f. 20, 21.
[41] Dufour ubi supra. Hor. Entomolog. 382.
[42] Male Insecta in some instances engender more than once. Mr. MacLeay sen. has observed this with regard to Chrysomela Polygoni, and I have noticed it in Bombyx Mori.
[43] Hor. Entomolog. 134. 200.
[44] Zoolog. Miscell. iii. t. 146. In this figure the segments are made much more distinct than they are in my specimen.
[45] Hor. Entomolog. 422—.