22 DEFINITION OF THE TERM INSECT. 



I shall regard as Insects all those Annulosa that respire 

 by tracheal and have no circulation, considering the 

 Trachean Arachnida and the Myriapoda for the present 

 as sub-classes, the one bordering upon the Arachnida, and 

 the other upon the Crustacea. Some of these I am ready 

 to own seem separated by an interval sufficiently wide 

 from the Hexapods, which may be regarded as more pe- 

 culiarly entitled to the denomination of Insects. The 

 most striking differences will be found in the coalition of 

 the head with the trunk in some (Phalangidcs), and the 

 disappearance of the annulose form of the body in others 

 (Acarus L.), so that the legs only are jointed". Yet an 



a There is some reason for thinking, though the octopod and my- 

 riapod insects breathe by trachea?, that there is no small difference in 

 thw distribution of these organs. The Trachean Arachnida have only 

 a pair of spiracles, from which the tracheae 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 situ- 

 ation 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 lulus, according to the observations of Savi ( Osservaz. per 

 servire alia Storia di una Specie de lulus, &c. 15 — ), consist of bundles 

 of parallel tracheae. 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 

 AraneidcB, which structure, probably, carries with it a system of cir- 

 culation, 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 CheliferidcE or Pseudo-Scorpions, and the Araneidce, to the other 

 Octopods ; and these analogies furnish a strong proof, that the Tra« 

 cheans belong rather to Insecta than Arachnida. Comp. N. Diet. 

 d'Hist. Nat. xxvi. 445 ; and Desertion de six Arachnid, nouv. &c, 

 par Leon Dufour, 16. 



b Mr. MacLeay observes with regard to the Tardigrade, de- 

 scribed by Spallanzani and Dutrochet, that " it proves that an animal 

 may exist without antennae 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 Gfeer, vii. t. vii,/. 14. 



