Dorothy J. Jackson 275 



seventh abdominal tergite is known as the propygidium, the eighth as 

 the pygidium. In both sexes the propygidium bears short bristles 

 posteriorly whilst the pygidium is covered with them. The preceding 

 tergites are devoid of bristles. On the ventral surface of the abdomen 

 in both sexes are five visible sternites. According to Hopkins' account 1 

 of the sternites of beetles of the genus Dendroctonus these five sternites 

 may be taken to represent sternites 3 to 7, as the first and second are 

 stated by him to be obscured by the coxal cavity and probably this 

 explanation holds good for Sitones also. The area on the sides between 

 the tergites and sternites is occupied by the pleurites. These consist of 

 a more dorsal line of membranous pieces, the epipleurites, which contain 

 the spiracles, and below them a line of chitinous pieces called the hypo- 

 pleurites. The pleurites are normally covered by the elytra. The external 

 sexual differences are to be found in the shape of the pygidium and the 

 hypopleurites and sternite of the last segment. 



The Male. (See Fig. II 2 , IV and VI, p. 276.) 



The pygidium is much larger than in the female and completely 

 overlaps the ends of the hypopleurites of each side. The hypopleurite of 

 the last segment is shorter than in the female and ends abruptly where 

 it touches the pygidium and is not continued round the end of the body 

 as a narrow edge dorsal to the sternite. The sternite of the last segment 

 has the edge truncated and not rounded. This point must be examined 

 carefully, for if hastily viewed with a lens the extremity of the sternite 

 of the male often appears as round as that of the female owing to the 

 rounded edge of the pygidium being closely applied to the sternite more 

 or less obliterating the anal orifice which occurs between them. 



The anal opening occurs between the pygidium and the seventh 

 sternite. 



The Female. (See Fig. I 2 , III and V, p. 270.) 



Pygidium much smaller than in the male and not overlapping the 

 hypopleurite. Hypopleurite of the last segment longer than in the male, 

 not ending abruptly but gradually narrowing to a small ridge which is 

 continued round the end of the body above the seventh sternite to join 

 with the hypopleurite of the other side. Seventh sternite with the 

 extremity evenly rounded. Anal orifice occurring between the pygidium 

 and the ridge of the hypopleurite. 



1 "The Genus Dendroctonus," by A. D. Hopkins. U.S. Dept. Agric. Bur. Ent. Technical 

 Series, No. 17, Bull, tin No. 83, Part 1, June 1909. 



- To simplify these illustrations I have omitted scales, bristles and sculpturing. 



