ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 357 



The sedoeagus, including penis, vesica, and ejaculatory duct, is common 

 to all insects, though represented in great variety, and is quite distinct 

 from the external clasping organs. The subject is finely illustrated. 



Homology of Body Setae in Caterpillars.* — Y. Hsuwen Tsou has 

 inquired (1) whether or not the setae on every body segment of any 

 Lepidopterous larva are serially homologous or not, and (2) whether or 

 not the setae on every corresponding segment of every Lepidopterous 

 larva are homologous to each other. He has established a nomenclature 

 for the body setae — anterior, dorsal, sub-dorsal, circumstigmatal, 

 lateroventral, pseudopodal, and mid- ventral. Each group is indicated 

 by the initial letter, and each seta by the letter and a number, D 1? P 4 , 

 and so on. 



The only reliable means of ascertaining the homology of the body 

 setae of Lepidopterous larvae is to associate the corresponding positions 

 which the setae occupy. The setae of any body segment are homologous 

 both with the setae of every other segment of the same larva and with 

 the setae of the same segment of different larvae. 



The setae may be increased in number and reduced in number as the 

 results of specialization. The thoracic setae of the larvae of the sub- 

 order Jugatae do not differ from the thoracic setae of the larvae of 

 generalized Frenatae in number, but do differ in arrangement of certain 

 groups of setae. The author regards Cossus cossus as the most 

 generalized of the Frenatae from the fact that it has no additional set* 

 and retains the first circumstigmatal (CJ on all the abdominal segments 

 and on the prothorax, while other Frenatae do not retain this seta. 



The abdomen of Lepidopterous larvae consists of more than ten 

 segments. The setae of the prothorax differ from those of the 

 mesothorax and metathorax, but they are more similar to the latter 

 than to the setae of any abdominal segment. The thoracic segments 

 are more generalized than the abdominal segments. The homology of 

 each group of setae is much more evident than the homology of the 

 individual seta within a group. 



Chromosome Groups in Genus Drosophila.f — C. W. Metz has made 

 an interesting study of the types of chromosome group in twelve species 

 of Drosophila, and finds indication of an evolutionary series. Five 

 species conform to one type in respect of their chromosomes, other four 

 conform to another type, and three to three additional types respectively 

 —making five types in all. In each type the chromosome group is 

 made up of certain different kinds of chromosomes, distinguished by 

 their size, form and behaviour. The mutaphase plates usually stand out 

 as clearly as in diagrams, and there seems no reason to doubt that every 

 chromosome has a character of its own, and that its individuality 

 persists from one generation to another. The phenomeno*n of the 

 actual pairing of chromosomes in the diploid groups is nowhere so 

 striking as in this genus. Each pair is composed of one maternal and 



* Trans. Amer. Micr. Soc, xxxiii. (1914) pp. 223-60 (4 pis.), 

 t Journ. Exper. Zool., xvii. (1914) pp. 45-58 (1 pi.). 



Aug. 18th, 1915 2 c 



