Sutures. — The sclerites constitute the greater part of the 

 body-wall, the soft membranous portions separating them 

 being in most cases narrow. Usually these narrow portions 

 are mere lines ; they are then called sutures. 



Frequently the sutures become entirely effaced. We are 

 therefore often unable to distinguish certain sclerites in one 

 species of insect which are distinct in another. 



In such cases the effaced suture is said to be obsolete. 



PARTS OF THE HEAD. 



The principal portion of the chitinized parts of the head 

 are firmly joined together so as to constitute a box which 

 contains what may be called by analogy the brain of the in- 

 sect and certain other important organs. To this are articu- 

 lated a number of movable appendages. The parts of the 

 head may be classed, therefore, under two divisions : first, 

 the fixed parts, or the skull ; second, the appendages. 



THE FIXED PARTS OF THE HEAD. 



Compound eyes. — The most striking in appearance of 

 tne fixed parts of the head are the eyes. These are two large, 

 nearly hemispherical bodies ; one on each side, forming a 

 considerable portion of the latero-dorsal part of the head. 



Study one of the eyes with a compound microscope, using 

 a low power. Note the honey-comb-like structure of the eye. 

 If you have difificulty in seeing this, remove a part of one eye 

 with fine-pointed scissors and mount it on a glass slip. Each 

 of the hexagonal divisions of the eye is a cornea of a distinct 

 eye. These large eyes are therefore compound, and each of 

 the small eyes of which they are composed is termed an 

 ommatidium (plural ommatidia).* 



" Formerly the ommatidia were termed ocelli (singular ocellus) ; but later 

 writers use the term ocelli only to designate the simple eyes. 



