ON THE DISTRIBUTION AND PRIMITIVE NUMBER 
5 OF SPIRACLES IN INSECTS.* 
BY A. S. PACKARD, JR. 
Pue TA 
Warme engaged in dissecting certain Sphinx and Bombycid 
larvæ, my attention was called to an interesting feature in the 
distribution of the thoracic. portion of the main tracheæ and their 
igmatal branches. In the larva of Sphinx and of Platysamia 
cecropia, and in fact so far as I am aware in all lepidopterous 
larvæ, there are nine pairs of spiracles, or stigmata, of whic 
eight are abdominal, there being a pair to each first eight seg- 
ments of the abdomen ; while there is but one pair of thoracic 
spiracles, which are invariably, so far as I am aware, situated on 
the prothoracic segment. On laying open the body of a Sphinx 
larva a large number of branches are seen to arise from the pro- 
thoracic and basal, or first pair of abdominal spiracles. Now 
between these two points it will be remembered that there are no 
Spiracles or any external signs of them. And yet the main tra- 
chea between these two spiracles deviates from its course and 
bends down to send off a small trachea to the place where, did a 
Spiracle exist, we should look for it, i.e., to a point in the suture 
between the mesothoracic and metathoracic segments, where in 
hy menopterous larva a spiracle does exist. From the upper side 
of the main trachea two larger branches are sent towards the 
‘terior of the body. These apparently correspond with the num- 
erous branches sent off from the spiracles. 
In Platysamia cecropia the same disposition of the main tra- 
chea May be seen, as it bends out in the same way towards the 
Usual site of” the spiracle in other groups of insects, and throws 
off three branches, one outward towards the tegument, small, and 
apparently rudimentary, while the two others, directed inwards, 
are larger than in Sphinx. 
: This has led me to ascertain how the spiracles are distributed 
a other groups of insects, and what is their usual number. 
While in the lepidopterous larvæ there is but one pair of stig- 
mata, which are situated on the prothoracic, or first thoracic, seg- 
*Read before the National Academy of Sciences, New York, en 
