ENTOMOLOGICAL NEWS. 29 



which is mixed with the water, are provided with so-called tra- 

 cheal gills. These are usually thin, thread-like, or plate-like 

 expansions of the body wall, richly furnished with tracheae. The 

 thin walls of these gills permit an interchange between the air in 

 the water and the air in the tracheae. Tracheal gills may be en- 

 tirely external, or may exist within the hind part of the intestine. 

 In that they are supplied only with tracheae containing air to be 

 purified, tracheal gills differ from true gills of fishes and crusta- 

 ceans, whose vessels contain blood to be purified. 



" The act of respiration consists in the alternate dilatation and 

 contraction of the abdominal segments, the air entering the body 

 chiefly at the thoracic spiracles. As in vertebrates, the frequency 



of the acts of breathing increases after exertion 'In the 



pupa and larva state, respiration is performed more equally by 

 all the spiracles, and less especially by the thoracic ones.' 

 (Packard) 



Insects are unisexual, i.e., the males and females are different 

 individuals.* No external characters, such as shape, color, etc., 

 can be given to distinguish between the sexes of all insects. The 

 reproductive organs are contained in the abdomen. Those of the 

 male are the testes, which are paired glandular bodies, secreting 

 the sperm, or fertilizing fluid, in which are numerous, very small, 

 tadpole-like spermatozoa. Each testis has a duct, the vas deferens, 

 Avhich may dilate into a vesicula seminalis, after which the two 

 vasa deferentia unite to form the ductus ejaculatorius, which opens 

 into the external intromittent organ (penis), usually situated on 

 the ventral side of the abdomen near its apex. 



The female reproductive organs are the pair of ovaries, in 

 which the eggs (ova) are found. The duct of each ovary (ovi- 

 duct) unites with its fellow and forms the vagina. The vagina is 

 connected with the ovipositor, f or has a free external opening 

 (vulva), usually situated on the ventral side of the abdomen near 

 its apex. ' ' Connected also with the vagina are one or more 

 pouches (receptacula semen's), within which the sperm is received 

 and stored." The sperm " retains its fertilizing properties for a 

 long time. Thus the queen bee or ant, pairs but once, though 



" Cases not unfrequently occur in which from arrest of development of the embryo, 

 the sexual organs are imperfectly developed, so as to present the appearance of being both 

 male and female" (Packard). Such individuals are termed hermaphrodites 



t See ENT. NEWS, vol. ii, p. 9. Opening into the vagina is sometimes a poison sac com- 

 municating with the sting. 



