122 INDIRECT INJURIES CAUSED BY INSECTS. 
may do no great injury; but where they abound, as they often do, by 
interrupting the course of the descending sap, and admitting wet be- 
tween the bark and wood, decay speedily ensues, and the tree perishes. 
Almost every kind of tree is Jiable to the assaults of one or more species 
of this tribe of insects. Even fruit-trees, as the apple, plum, &c., have 
«each their Scolytus ; and at Rouen I found a species, I believe unde- 
scribed, which teeds on the mountain ash. It is to our large forest-trees, 
however, that they are most injurious. Thus the common ash is assailed 
by Hylesinus frawini, the pinnated labyrinths of whose larve you can 
hardly fail to observe on the first piece of loose bark you detach from 
the rough-split posts and rails made of this wood ; while the bark-borer of 
the oak is a small beetle of an allied genus, Scolytus pygmaeus, which with 
us does no great harm, but so abounded of late years in the Bois de 
Vincennes, near Paris, that 40,000 trees were killed by it ; and many of 
the finest elms in St. James’s Park and Kensington Gardens?, as well as 
in the promenades of various cities in the north of France, have fallen 
victims to another of this tribe, Sco/ytus destructor, whose trivial name well 
characterises the frequency and severity of its ravages.? 
1 MacLeay in din. Phil. Journ. xi. 123. ' 
2 While residing at Brussels in the spring of 1836, having pointed out to Dr. 
George, Professor of Botany at the University, that many of the elms in the park 
were infested with this insect, and that there was imminent risk of this noble pro- 
menade, which consists almost wholly of elms, being destroyed by it, he brought the 
subject under the notice of the burgomaster and municipal council, who very wisely 
had the diseased trees cut down, as well as the many much younger but equally in- 
fested trees of the Boulevards, and the bark of the whole peeled off and carefully 
burnt. I afterwards found, in a tour along the north coast of France through Nor- 
mandy, &c,, that the elms in the promenades (almost always formed of this tree), in 
all the large towns, were in a course of rapid destruction by this same Scolytus 
destructor, particularly at Calais, Boulogne, Rouen, Havre, and Caen; and numerous 
observations convinced me that the general opinion that these insects attack only 
those trees which are previously diseased from natural decay is altogether erroneous, 
and that Professor Audouin’s discovery is as important and correct as novel — 
namely, that though it is quite true that the female Scolyti never lay their eggs 
except in trees which are in a declining state; yet it is equally certain that the 
healthiest elms, where Scolyti abound, are constantly brought into this languishing 
state by the attacks of the males, or, as M. Audouin conceives, of both sexes (see 
remarks on this point by W. Spence in Zrans. Lnt. Soc. Lond. ii. proc. xly.), upon 
the bark for food; so that in consequence of the loss of sap from the numerous holes 
which they gnaw, and the subsequent mischief from the rain penetrating into them, 
the trees are soon brought into that unhealthy condition which the instinct of the 
female requires to induce her to lay her eggs in them. (Spence in Zvans, Ent. 
Soc. Lond. ii. proc, xiii, xv. xx. xxy.; Audouin in Ann. Wnt. Soc. de France, Bull. 
Jan, 4, 1837; Silbermann, Rev. Mntom. iv. 115., where Dr. Ratzeburg is quoted as 
stating that the large weevil (Pissodes notatus) in like manner attacks the bark of 
young pines with its trunk, and thus renders the trees unhealthy before the female 
deposits her eggs in them.) For a further description of the mischief done by 
Scolytus destructor, and the means of preventing its extension, see a communication 
by W. 8. under the article U/mus, in Mr. Loudon’s Arboretum et Fruticetum Britan- 
nicum; to which admirable work the reader is also referred for more complete details 
than could be here given in the valuable contributions by Mr. Westwood relative to 
insects injurious to this and other species of forest- trees. 
Tt may be here mentioned, though somewhat out of place, for the purpose of draw- 
ing the attention of Entomologists to a new tribe of insect-parasites of which no 
account appears to have been given in books, that in examining closely the pup of 
Scolytus destructor at Brussels, 1 found them lined in different parts of their external 
surface, but especially on the thorax and about the cases of the elytra, with numer- 
