498 EXTERNAL ANATOMY OF INSECTS. 
male flies and the drone. ‘The male, likewise, of more 
than one species of Ephemera, besides the common 
lateral eyes and the stemmata on the back of the head, 
have a pair of compound eyes on the top of a short co- 
lumnar process* In the Hemzptera Order, also, an 
instance occurs of four eyes in the genus Aleyrodes>. 
Amongst the vertebrate animals, there is an example of 
eyes with two pupils in Anableps, a genus of fishes °, but 
no vertebrate animal has fowr of these organs. That 
many insects should have more than ¢wo eyes, will not 
seem to you so extraordinary as that any should be found 
that, like the Cyclops of old, have only one. There is, 
however, an insect, before celebrated for its agility 4 
(Machilis polypoda), which has a single eye in its fore- 
head; or we may say, its eyes are confluent, without any 
line of distinction between them except a small notch 
behind. Now that I am treating of the number of eyes, 
I must not forget to observe to you, that in some insects 
no eyes at all have been discovered. In Polydesmus com- 
planatus, on each side of the head there is an eye-shaped 
portion separated by a suture, in which under a power- 
ful lens I cannot satisfy myself that I can discern any 
thing like the facets that usually distinguish compound 
eyes. In Geophilus electricus, another myriapod, they 
certainly do not exist. Whence we may conclude, as 
was before observed‘, that the faculty of emitting light 
is rather given it as a means of defence than to guide it 
in its path. 
> Prate XXVI. Fic. 39. h. 
b Latreille Gen. Crust. et Ins. iii. 73. 
© N. Dict. d’ Hist. Nat.i. 479. 4 Vor. II. 316. 
© De Geer vii. 562. f Vor. II. p. 226. 
