22 DEFINITION OF THE TERM INSECT. 
I shall regard as Insects all those Annulosa that respire 
by trachez? 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 (Phalangide), and the 
disappearance of the annulose form of the body in others 
(Acarus L.), so that the legs only are jointed’. Yet an 
* There is some-reason for thinking, though the octopod and my- 
riapod insects breathe by trachez, that there is no small difference in 
the distribution of these organs. The T’rachean Arachnida have only 
a pair of spiracles, from which the trachez 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 PLarr X XIX. Fic. 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 Iulus, according to the observations of Savi (Osservaz. per 
servire alla Storia di una Specie de Iulus, &c. 15—), consist of bundles 
of parallel tracheze. 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 
Araneide, 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 Cheliferide or Pseudo-Scorpions, and the Araneide to the other 
Octopods ; and these analogies furnish a strong proof, that the T’ra- 
cheans 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. 
> Mr. MacLeay observes with regard to the Tardigrade, described 
by Spallanzani and Dutrochet, that “it proves that an animal may 
exist without antenne or distinct annular segments to the body, but 
having two eyes and six articulate legs.” (Hor. E’ntomolog. 350—.) 
Many Acari prove the same thing. De Geer, vii. t. vii. f. 14. 
