66 EXTERNAL ANATOMY OF INSECTS. 



the top or crown of the skulh The Giila, or throat, is the 

 portion immediately below the under lip, and extends to the 

 union of the head with the prothorax. The fore-part of the 

 throat is sometimes called the mentum, but has no fixed limit. 

 The mentum of MacLeay is the labium of Fabricius. 



The parts of the skull are these : the crown, the two eyes, 

 the throat. These are the four divisions of a section. 



I would propose these names for the — 



FIXED PARTS OF THE HEAD.P 



(^) Epicranium, ox upper part of the slcull. 



(se ) Clypeus, or shield of the mouth. 



(oe) Ocelli, ox simple eyes. 



{yE) GuLA, or throat, 



{ce) Mentum, ox chin. 



{(E) OcuLi, or eyes. 



MOVEABLE PARTS OF THE HEAD. 



(y) Antenna, ox cranial feelers. 

 ( j ) Os, or mouth. 



As before observed, the great development of one part 

 necessarily requires the proportionate diminution of another 

 part. A part increases or decreases in volume precisely as the 

 organs it may bear require muscle for their guidance and 

 government; on the (acknowledged) plan, that, for so much 

 muscular exertion so much muscle must be provided, which 

 muscle must occupy so much space. This is well illustrated 

 by the head of insects. Professor Sang has prettily observed, 

 that every instrument, whether it be for the generation or 

 transference of power, has a best size and a best form. Nature, 

 in the formation of her instruments, has always adopted that 

 best size and best form. If her creatures wanted but to see, 

 a globular eye floating in space might perhaps be the uniform 

 character of the animal world. If to see and to eat, an eye 

 and a mouth would be given. If to move swiftly in the air 

 were desirable, wings must be supplied ; if, on the earth, legs 

 must be added ; if in the water, fins. To carry all these organs, 



p In this table the parts marked with small diphthongs are variable and 

 inconstant ; those with capital diphthongs are constant throughout the classes. 

 The diphthongs and letters refer to figures in Plate V. 



