EXTERNAL ANATOMY OF INSECTS. 509 
this last particular I shall enlarge hereafter. Their 
structure, as far as it differs in the sexes, I fully dis- 
cussed in a former letter?; and the most remarkable 
kinds of them will be included in a set of definitions 
which I shall draw up for you before our correspondence 
on this part of my subject closes: I shall therefore now 
confine myself to the following particulars—namely, 
their number, insertion, substance, situation, proportion, 
general form and structure, clothing, expansion, motions, 
and station of repose. 
As to their Number, in the majority of crustaceous ani-~ 
mals the antennz amount to four, but no zasect has more 
than ¢wo. A remarkable hemipterous genus (Ofdocerus °) 
seems to afford an exception to this rule, since the 
species composing it at first sight appear to have four, 
and in some instances even szz antennze; but as only /wo 
of them terminate ina bristle, the other, though pro- 
ceeding from the same bed of membrane, may perhaps 
be regarded as merely appendages. Germar, who has 
described a species of this genus* under the name of Co- 
bax Wintheri, considers these appendages as analogous 
to palpi: but as they do not proceed from the oral or- 
gans, but from the bed of the antenna at the base of the 
nose’, they ought certainly to be regarded rather as ac- 
cessories to the latter, than as representing the former. 
In the Aptera Order the mites (Acarina) appear to be 
without these organs. In the pupiparous tribe Hzppo- 
bosca they seem about to disappear; and in the Arach- 
* See above, p. 316—. > Linn. Trans, xi. 
© Mag. der Entomolog. iv. 5. ‘ 
* Palpi quatuor, subzequales, cylindrici, ad basin elypet.— Germ, 
