ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 213 



Insects and Disease.* — A very useful guide has been prepared 

 dealing with the specimens and models exhibited in the Central Hall of 

 the British Museum (Natural History), which illustrate the importance 

 of insects in connexion with the spread of disease. The guide deals 

 with mosquitoes and malaria, mosquitoes and yellow fever, mosquitoes 

 and filariasis, fishes that feed on the larvae of mosquitoes, tsetse-flies 

 and trypanosomiasis, Tabanid flies and " Calabar swellings," house-flies 

 and intestinal diseases, sheep bot-fly, fleas and plague, bed-bugs and 

 disease, lice and disease, ticks and disease. 



Chromosome Studies in Diptera.t — Charles W. Metz has studied 

 the chromosomes of about eighty species of Diptera with especial 

 reference to the phenomenon of "chromosome pairing." In all cases 

 the chromosomes were found to be uniformly associated in pairs in 

 diploid cells, whether somatic or germinal. The paired association was 

 found to continue throughout all stages of cell-division from earliest 

 prophase to latest anaphase, being most intimate in the earliest and 

 later stages. Association of paternal and maternal chromosomes 

 apparently is effected in early cleavage stages (perhaps before the first 

 cleavage), since in the late cleavage stages the chromosomes are definitely 

 paired. The paired association was found to continue during all stages 

 in ontogeny from the egg to the adult. 



Certain cases of multiple chromosome numbers (tetraploid or higher 

 multiples) were found in occasional cells. In these cases corresponding 

 chromosomes were associated together in prophase in aggregates of four, 

 eight, etc., instead of being arranged in pairs. In many species several 

 pairs of chromosomes (in some cases nearly all) could be individually 

 distinguished by characteristics of size and form. In a species of 

 Drosophila each pair of chromosomes is very clearly differentiated from 

 all others. These individually marked pairs of chromosomes, with the 

 exception of the sex-chromosomes in males, were in all cases symmetrical, 

 i.e. composed of similar members. In certain respects the pairing 

 phenomena were found to present a striking similarity to synaptic 

 phenomena. They give an actual demonstration of a side-by-side 

 approximation of corresponding chromosomes. 



The facts lend strong support to three conclusions : (1) that the 

 paired arrangement of chromosomes is not due to a random assorting 

 process, but is selective to the highest degree ; (2) that each maternal 

 chromosome becomes associated with a definite, similar paternal chromo- 

 some and with no other ; and (3) that chromosome pairing is dependent 

 upon the qualitative nature of the chromosomes — and more specifically 

 upon a qualitative (physico-chemical) similarity between associating 

 members. 



Viviparity in Diptera. % — D. Keilin divides those Diptera which 

 are always viviparous into two groups. First, there are those in which 

 the larvse are not nourished in the uterus of the mother, where only the 



* British Museum (Nat. Hist.) 1916, Special Guide No. 7, 45 pp. (14 figs.), 

 t Journ. Exper. Zool., xxi. (1916) pp. 213-79 (8 pis.). 

 X Arch. Zool. Exp6r., Iv. (1916) pp. 393-415 (8 figs.). 



