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 Lihellula 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 (.'.); hut 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 larvae 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 very 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 tlie 

 suctorial, or the Mandibulate and Ilaastellate mouth ; 

 in the first the mouth is formed for prehension and 

 biting, as in the Coleoptera or beetles ; in the second 



