The Nucleolus and IVlicrochromosomes in the 
Spermatogenesis of Hydrophilus piceus. (Linn.) 
By 
George Arnold. 
University of Liverpool. 
With plate IX — XI. 
In the followiug pages it is proposed to describe the somatic 
aud Ist. and 2nd. meiotic divisions in the spermatogenesis ofHy- 
drophiliis piceus. I have also followed up the changes to the 
formation of the spermatozoa, the results of which, it is hoped, will 
be published in a subsequent paper, especially in view of the fact 
that Hydrophilus differs materially from Cybister, as described 
by Voixov ('02), which is not a distantly related species. 
Recently Wilsox (’Oö) published the results of an examination 
of a large number of Hemiptera, results of a very startling character. 
The importance which he attached to the presence of an accessory 
(•‘heterotropic”) and unpaired chromosome , and of idiochromosomes 
of the Ist. meiotic division, in the determination of sex, and their 
bearing on Mendelian phenomena, rendered it very desirable that 
further investigations should be made, not only on other members 
of the Hemiptera, but on those of other Orders in the Insecta. 
Two seasons of careful and assiduous collecting failed to obtain 
sufficient numbers of the larger Hemiptera found in England, for 
preparation and staining. Under these circumstances, attention was 
turned to the subject of this paper. 
Hydrophilus piceus is fairly plentiful, if local, in the South 
of England, and early in the year, at the beginning of May, speci- 
mens were obtained from a dealer in aquaria requisites. 
The beetles were kept in a large aquarium under as natural 
conditions as possible i), with an abundance of green food, and after 
a week or so were killed and fixed. 
q Several females made eocoons, out of which all the larvae were suc- 
cessfully hatched. 
