34 STUDIES IN SPERMATOGENESIS. 



COLEOPTERA. 



Trirhabda virgata (Family Chrysomelidae). 



Two species of Trirhabda were found in larval, pupal, and adult 

 stage on Solidago sempervirens , one at Harpswell, Maine, the other at 

 Woods Hole, Massachusetts. The adult insects of the two species 

 differ slightly in size and color, the germ cells mainly in the number 

 of chromosomes, Trirhabda virgata having 28 and Trirhabda cana- 

 dense 30 in spermatogonia and somatic cells. 



In Trirhabda virgata , the metaphase of a spermatogonial mitosis 

 (plate vin, fig. 3) contains 28 chromosomes, one of which, as in Tene- 

 brio molitor is very much smaller than any of the others. The mater- 

 nal homologue of the small chromosome is, as later stages show, one 

 of the largest chromosomes. In Tenebrio the unequal pair could not 

 be distinguished in the growth stages of the spermatocytes. In 

 Trirhabda it has not been detected in the synizesis stage (fig. 4), but 

 in the later growth stages (figs. 5-7) this pair is conspicuous in 

 preparations stained by the various methods cited above, while the 

 spireme is pale and inconspicuous. The size of the heterochromosome 

 pair varies considerably at different times in the growth period, and 

 in some nuclei (fig. 7) both chromosomes appear to be attached to a 

 plasmosome. The ordinary chromosomes assume the form of rings 

 and crosses in the prophase of the first maturation mitosis (fig. 8), but 

 usually appear in the spindle as dumb-bells or occasionally as tetrads 

 (fig. 10), or crosses (fig. 1 1). The unsymmetrical pair is plainly seen in 

 figures 9 and n, but is not distinguishable in a polar view of the meta- 

 phase (fig. 13). In the anaphase (figs. 14-16) the larger and the smaller 

 components of the pair separate as in Tenebrio. This is, therefore, 

 clearly a reducing division as far as this pair is concerned, and prob- 

 ably for all of the other pairs, though neither the synapsis stage nor the 

 prophase forms are so clear on this point as in some of the other species 

 studied. Figures 17 and 18 show metaphases of the two classes of 

 second spermatocytes, the chromosomes varying somewhat in form in 

 different preparations and even in different cysts of the same prepara- 

 tion. An early anaphase of this mitosis is shown in figure 19; here 

 the small chromosome is already divided. It was impossible to find 

 good polar views of the daughter plates in the two classes of second 

 spermatocytes, but it is evident from figure 19 and other similar views of 

 the second spermatocyte spindle that, as in Tenebrio, one-half of the 

 spermatids will contain one of the derivatives of the small chromo- 

 some, the other half one of the products of its larger homologue. 



