ZOOLOGY. ` 183 
true insects, ania thirteen, though it may fall short of this num- 
ber, as in the larve of Hydrophilide which have but twelve. 
; This fact is plainly seen in all insects undergoing complete meta- 
morphoses, where the head constitutes one, the thorax three and 
the abdomèn nine joints. In some insects ugdergoing incomplete 
metamorphoses, and notably in Libellulide, an apparent tenth ab- 
dominal joint is visible ; but’ Dr. Schaum, in the article alluded to, 
has very conclusively:shown that what is generally mistaken for 
the first abdominal joint is but a posterior portion of the meta- 
thorax, and I know from conversation with, and from notes and, 
correspondence of my late friend Walsh, who gave this question 
much study, that he was of the same opinion. + A more or less 
distinct terminal subjoint is also often noticeable at the extremity 
of the body in many larve, and I especially called attention to 
this fact, when making the statement criticised, and cited as a 
prominent example, the larva of Passalus cornutus.* 
In reality, as Erichson and Stein have proved, this is nothing 
but the externally protruded anus, analogous to the anal proleg of 
the laryee of many Coléoptera. The fact that dipterists have 
characterized the Cecidomyidous larva as differing from all other 
insect larvee in having fourteen joints shows how universally the 
insect body is considered 13-jointed; and I have already stated 
my belief, { after examination of many species, that these larvae 
form no exception to the rule of having thirteen joints and a sub- 
_ joint. Strictly speaking, therefore, the body of an insect is com- 
- posed of thirteen joints and a subjoint; and if we wish to em- 
ploy a more arbitrary definition, the number 13 will more truly 
and generally apply than either 12 or 14. 
(3) I have shown above that I do not believe this tò be a 
_ truth; and even if it were irrefutably demonstrated that the head 
-of an insect is composed of four elementary or embryonic joints, 
TI should still speak of it as a single joint in referring te an insect 
out of the egg; for nothing would be gained, éspecially in a pop- 
ular work, in which the abstruse in thought or expression: should 
always be avoided, by substituting the ideal for the real. Though 
the petals of a flower be modified leaves, we still distinguish them 
as petals; and he who would — to do away with all the dis- 
c 
~ 
= 35th Bo. ep p. 114, ote : 
