THE EXTERNAL ANATOMY OF INSECTS 37 



that is to say nymphs and naiads possess compound eyes. But the 

 larvae of insects with a complete metamorphosis, except Corethra, do 

 not possess well-developed compound eyes; although there are fre- 

 quently a few separate ommatidia on each side of the head. These 

 are usually termed ocelli; but the ocelli of larvae should not be con- 

 fused with the ocelli of nymphs, naiads, and adults. 



The corneas of the ocelli. In addition to the compound eyes most 

 nymphs, naiads, and adult insects possess other eyes, which are 

 termed ocelli. The cornea of each ocellus is usually a more or less 

 nearly circular, convex area, which is not divided into facets. The 

 typical number of ocelli is four; but this number is rarely found. 

 The usual number is three, a median ocellus, which has been derived 

 from a pair of ocelli united, and a distinct pair of ocelli. Frequently 

 the median ocellus is lacking, and less frequently, all of the ocelli 

 have been lost. The position of the ocelli is discussed later. 



b. THE AREAS OF THE SURFACE OF THE HEAD 



In descriptions of insects it is frequently necessary to refer to the 

 different regions of the surface of the head. Most of these regions 

 were named by the early insect anatomists; and others have been 

 described by more recent writers. 



This terminology is really of comparatively little morphological value; for 

 in some cases a named area includes several sclerites, while in others only a portion 

 of a sclerite is included. This is due to the fact that but few of the primitive 

 sclerites of the head have remained distinct, and some of them greatly over- 

 shadow others in their development. The terms used, however, are sufficiently 

 accurate to meet the needs of describers of species, and will doubtless continue in 

 use. It is necessary, therefore, that students of entomology become familiar 

 with them. 



The best landmark from which to start in a study of the areas of 

 the surface of the head is the epicranial suture, the inverted Y-shaped 

 suture on the dorsal part of the head, in the more generalized insects 

 (Fig. 45, e. su.). Behind the arms of this 

 suture there is a series of paired sclerites, which 

 meet on the dorsal wall of the head, the line of 

 union being the stem of the Y, a median suture; 

 and between the arms of the Y and the mouth 

 there are typically three single sclerites (Fig. 45, 

 F, C, L). It is with these unpaired sclerites 

 that we will begin our definitions of the areas 

 of the head. Fig. ^S^-Head fa 



The front. The front is the unpaired 

 sclerite between the arms of the epicranial suture (Fig. 45, F). 



