24 ENTOMOLOGY FOR MEDICAL OFFICERS 



segments are, as a rule, quite distinct, and they often over- 

 lap one another; the number of abdominal segments is 

 not constant, the maximum in any adult insect is lO, but 

 in most insects it is much fewer than this. In insects with 

 very few visible abdominal segments it is often the case 

 that several terminal segments are modified to form an 

 ovipositor, which, when it is not in use, is usually invaginated 

 and invisible. 



The Head and its Appendages. — The head contains, 

 besides appropriate muscles, the supra-oesophageal ganglion, 

 or brain, and the pharynx ; its appanages and its true 

 appendages are sensory and feeding organs. 



At the sides of the head are the eyes ; on the under- 

 surface is the mouth, flanked by the mouth-appendages, 

 or trophi ; above the mouth, extending across the face, is 

 the clypeus ; the upper part of the head is the front, or 

 epicranium ; the back of the head, abutting on the thorax, 

 is the occiput; in elongate heads the ventral area behind 

 the mouth is known as the gule. 



The appanages of the head, as distinguished from the 

 true paired appendages, are the following : — 



(i) Depending or projecting from the free edge of the 

 clypeus is the labrum, or upper lip, a sort of eave overhanging 

 the mouth ; in many insects that prick and suck, the labrum 

 is produced to form (2) a long style known as the epi- 

 pharynx; (3) in the floor of the mouth is a process on 

 which the salivary ducts open; this when it is soft is 

 known as the tongue, but when it is produced far beyond 

 the mouth, as it is in flies, is called the hypopharynx; the 

 other appanages of the head are (4) the lateral eyes and 

 the ocelli. The lateral eyes are generally compound and 

 faceted, but they may be non- faceted, or they may be 

 absent altogether ; the ocelli or simple eyes, when present, 

 are usually three in number, and stand in a triangle on 

 the crown of the head; but sometimes there are only 

 two ocelli. 



The true appendages of the head are the antennse and 

 three pairs of jaws. The antenncB are generally placed 

 between the eyes; though commonly elongate, flexible 

 and many-segmented, they vary greatly in details of 



