146 MEDICAL ENTOMOLOGY 



first part of its period of growth, until its posterior end, which bears 

 the stigmal plates, comes to lie in the vulva, through which its supply 

 of air is drawn in. 



There are two pairs of milk glands, both in Hippobosca and in 

 Melophagus ; one pair in the latter is rudimentary, and does not 

 assist in providing nourishment for the larva. The functional glands 

 resemble those of Glossina so closely both in their structure and 

 disposition as to call for no further remark. 



The Spermatozoa of Diptera are long filiform bodies (Plate XXIX, fig. 

 6), which frequently exhibit active lashing movements. At one end there 



is a slight swelling containing the nucleus, which is 

 The Spermatozoa , . . . . _. . 



difficult to stain properly. They do not present many 



differences in the different families, and their only importance from the 

 present point of view lies in the possibility that they may be mistaken for 

 parasitic organisms, such as spirochaetes or filaria. They are frequently 

 seen free in dissections of male insects, having escaped through the 

 rupture of the testes or the seminal vesicle, or in the female through 

 rupture of the spermathecae. When once recognized, however, they 

 are not likely to be mistaken for anything else. 



The spermatozoa arise from the germinal cells of the testes by a 

 process corresponding to that which produces ova in the female. Here- 

 ditary transmission of parasites through the male has not yet been 

 suggested in insects. 



THE NERVOUS SYSTEM 



A brief note on the nervous system will suffice here, as all that is 

 necessary for the parasitologist is to be able to recognize the parts when 

 met with in sections and dissections. 



The nervous system of an insect consists of ganglia and nerves ; the 

 latter, as in the vertebrates, are either motor or sensory. The arrange- 

 ment of the parts in the various groups is best under- 

 Oevelopment 



stood by a reference to their origin. In a very simple 

 insect there would be, theoretically, one ganglion, giving off motor nerves 

 and receiving sensory nerves, on each side of each segment, and such 

 a condition is in fact approached in some known forms. In later forms the 

 ganglia have become fused with one another to a greater or less extent ; 

 the fusion results, as it has done in the vertebrates, in the gradual 

 accumulation of the ganglia in the anterior part of the body, and par- 

 ticularly in the head. Each segment of the head may be supposed 

 to have, had originally its. own pair of ganglia for the control of its own 



