714 EXTERNAL ANATOMY OF INSECYS. 
spiracles; these in Epeira Diadema are three pair of 
small black points: on the back of the abdomen also 
are four pair, but in some species there are only two?: 
the most remarkable, however, are exhibited by the can- 
criform spiders before noticed®: in Carkinodes cancri- 
formis, in the plate which covers the abdomen, they are 
dark red spots with an elevated rim and centre‘ exactly 
resembling spiracles, except that they are not perfo- 
rated ; there are twenty-four of them, twenty arranged 
round the margin, and four in a square in the disk. 
3. Organs of motion. In a former letter you were 
told that several insects are enabled to leap by means 
of organs in their abdomen; I shall now describe such 
of them as require further elucidation. I then said that 
Podura and Sminthurus, two apterous genera, take their 
leaps by means of an anal fork’. In the former genus 
the fork consists of a single piece attached to the under 
side of the anus, and terminating in a pair of long slender 
sharp processes which articulate with it and form the 
fork or saltatorious instrument &. In Sminthurus the tines, 
as they may be called, of the fork do not articulate with 
the base, but are of the same piece and consist of two 
joints, the terminal one being flat and obtuse f. Machilis 
to the anal fork adds eight pair of ventral linear springs 
(Elastes), which are covered with hair or scales, and ter- 
minate in a bristle or two. I have on a former occasion 
mentioned the natatorious laminz with which the anus 
Treviranus. Arachnid. 23—. 
» See above, p. 705. 
° Pra're XXIX. Fic. 26. represents one of them. 
* Vou. IL. p. 315—. 
¢ Prate XV. Fre. 14. MM”. De Geer, vii. ¢. i. f. 5, 10, 21. 
f Ibid. ¢. iii. f. 4, 14. | 
