178 THE SCOTTISH NATURALIST 



gigas in my collection measures from front of head to end of 

 eighth dorsal segment just under 38 m^m., others reaching 

 36 and 35 mm. ; while the two smallest measure 18 and 

 24 mm. The largest cyaneus female is 30 mm., while the 

 smallest is no more than 16 mm. 



{p) Time of Emergence. — During July and August Sirex 

 gigas usually gnaws its way out of the pupal chambers under 

 the bark or thin external layer of wood of the conifer in 

 which it has spent its two or three years of larval life and 

 subsequent stages of development ; but many delay till 

 September, while late ones may be seen in October, and an 

 occasional exceptionally early one in June. Of seventy 

 records in which the month is given — I have many more in 

 which only the year is forthcoming — the numbers are appor- 

 tioned as follows : — June, one record ; July, fifteen ; August, 

 thirty-eight ; September, fourteen ; and October, two. If 

 my records of cyaneus are of any significance on this point, 

 it is on the whole a rather later insect to appear. Of twenty- 

 eight dated records, two are in June, two are in July, ten are 

 in August, eight in September, and six in October. Records 

 of specimens obtained during winter and spring are due to 

 artificial disturbance. An interesting figure of vS. noctilio 

 {? cyaneus) gnawing its way out of a piece of pine wood is 

 given in Dr James Ritchie's recent book on The Influence of 

 Man on Animal Life in Scotland (p. 468). As giving some 

 idea of the extent to which a tree may in the course of time 

 become infested by cycles of 5//uv larvae, it may be mentioned 

 that in May 1922 I examined a dead silver fir lying in a 

 plantation near Penicuik with the following results : — For 

 8 or 9 feet from the root no flight holes were observed, 

 but from that point to where the top of the tree had been 

 broken off — a distance of about 15 feet, with an average 

 girth of about 3! feet — I counted on the exposed two-thirds 

 of the surface of the trunk no less than 360 exit holes. 

 Assuming that the third of the circumference next the 

 ground was similarly pierced, the total flight-holes must thus 

 have reached close on five hundred, from each of which a 

 Sirex (or its parasite) had at some time or other emerged. 

 Fortunately I was able to find several of the insects dead 



