10 ENTOMOLOGY FOR MEDICAL OFFICERS 



the plasma is greenish or bluish, and in a few the plasma is 

 red from dissolved haemoglobin. 



The heart can be seen in action, with a minimum of trouble, by 

 watching a " blood- worm " (larva of Chtronomus, p. 121) under the 

 microscope. 



The central nervous system consists of (i) a large supra- 

 oesophageal ganglion or brain, lying dorsally in the fore 

 part of the head, and (2) a chain of ganglia extending 

 ventrally along the body. The brain and the foremost 

 (suboesophageal) ganglion are connected by a pair of com- 

 missures which embrace the gullet like a collar. In Arthropods, 

 such as Centipedes and insect-larvae, where the segments 

 behind the head are all much alike, the ventral chain is 

 simple and regular, with a ganglion for every segment ; but 

 in crabs and many flies the ventral ganglia are concentrated 

 in the thorax ; and between these extremes there are many 

 gradations. 



The nervous system can be seen, with a minimum of 

 trouble, in any transparent aquatic insect-larva; it is 

 particularly easily seen in the larva of Siinulium (Fig. 31), 



The senses of Arthropoda are highly developed. The 

 antennae are not only — like the appendages — organs of touch, 

 but are also organs of special sense, particularly of smell. 

 Special hairs near the mouth and on the mouth-parts and 

 their palps may possibly be organs of taste, since many 

 Arthropods show strong predilections in the matter of food. 

 Undoubted sound-conducting organs, besides mere vibratile 

 (auditory) hairs, are present in certain insects (organs for the 

 production of sound being extremely common). The eyes of 

 Arthropods are of two kinds— simple eyes, or ocelli (Lat. 

 ocellus = 3. little eye), and compound or faceted eyes — both 

 kinds often being present in the same individual. Both 

 kinds of eyes are composed of special translucent, refractive, 

 perceptive, and pigmentary elements. The simple eye, or 

 ocellus, consists of an epidermal pit, or cup, of cells lying 

 below a cuticular cornea-lens. The cells of the bottom of the 

 cup form a retina, the transparent ends of the cells of the 

 sides of the cup form a vitreous body, and the cells of the 

 edge of the cup are pigmented to form a sort of diaphragm. 



