6 
INTRODUCTION. 
the darted hand is seen by the insect. The number of 
facettes varies, some insects having as many as 25,000, 
as in Mordella , a small kind of beetle, others upwards 
of 17,000, as in Papilio , a genus of butterflies, others 
12,000, as in Libellula or dragon-fly; the house-fly has 
about 4,000, and the ant only 50 facettes; the simple 
eyes, ocelli or stemmata , as they are sometimes called, 
are generally three in number, and arranged on the 
forehead in a triangular form thus (,*,); but they do 
not exist in all insects; they are nearly always black, 
round, and more or less convex. How far these ocelli 
differ functionally from or are supplemental to the 
large compound eyes, it is not possible to say with 
certainty ; it has been suggested that they are intended 
for the perception of near objects, such as the various 
organs and pollen-producing parts of plants; as their 
refractive power is great, this is probable enough. These 
ocelli resemble those of the Arachnida, which do not 
possess compound eyes. All larvrn of insects which go 
through a complete metamorphosis possess only single 
eyes ; the composite facetted organs are developed late 
on in the pupal stage. 
The mouth is a ver}^ important and interesting point 
in the organization of an Insect; its structure is sub¬ 
ject to almost infinite variety, though a common type 
underlies all the various forms, the same organs, how¬ 
ever, being sometimes so modified in appearance, as to 
be with difficulty recognizable. Two chief types or 
plans are seen in insects, the masticatory and the 
suctorial, or the Mandibulate and Haustellate mouth ; 
in the first the mouth is formed for prehension and 
biting, as in the Coleoptera or beetles ; in the second 
