156 INDIRECT INJURIES CAUSED BY INSECTS. 



though much smaller, has similar habits, and is injurious by boring into 

 apple, pear, and walnut trees. 



The insects which attack the bark of trees mostly belong to the family 

 of Scoli/tidce Westwood (including the genera Scolyius, Hylesinus, Hylur- 

 gus, Tomicus, &c.) ; a numerous tribe of beetles, the larvae of which, 

 after being hatched from the eggs deposited by the parent beetle, excavate 

 in the substance of the inner bark, and partly also in the adjoining albur- 

 num or sap-wood, laterel parallel channels more or less sinuous, proceeding 

 on each side from a central one (that in which the eggs were placed), and 

 thus giving to the under side of the detached bark and exposed alburnum 

 that pinnated labyrinthine appearance, and fancied resemblance to letters, 

 which made Linne affix to one of these insects, to be presently alluded 

 to, the trivial name of Typographus. When in small numbers these 

 larvae may do no great injury ; but where they abound, as they often do, 

 by interrupting the course of the latex, or descending sap, and admitting 

 wet between the bark and wood, decay speedily ensues, and the tree per- 

 ishes. Almost every kind of tree is liable to the assaults of one or more 

 species of this tribe of insects. Even fruit-trees, as the apple, plum, &tc., 

 have each their Scolytus ; and at Rouen I found a species, I believe 

 undescribed, which feeds 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 fraxini, the pinnated labyrinths of whose larvae 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 af an allied genus, Scolyius pygmctus, 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, Scolytus destructor, whose trivial name 

 well characterizes the frequency and severity of its ravages.^ 



' MacLeay in Edin. 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 eminent risk of this noble promenade, which consists almost 

 wholly of elms, being destroyed by it, he brought the subject under the notice of the bur- 

 gomaster and municipal council, who very wisely had the diseased trees cut down, as well 

 as the many much younger but equally infested trees of the Boulevards, and the bark of 

 the whole peeled ofl' and carefully burnt, I afterwards found, in a tour along the north 

 coast of France through Normandy, &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 erroneoi;^, 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 ahoxxnA, 

 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 Trans. Ent. 

 Sac. Land. ii. proc. xlv.), 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 

 mstinct of the female requires to induce her to lay her eggs in them. (Spence in Trans. 

 Ent. Soc. Lond. ii. proc. xiii. xv. xx. xxv. ; Audouin in Ann. Ent. Soc. de France, Bull. Jan. 

 4. 1837 ; Silbermann, Rev. Entom. iv. 115., where Dr. Ratzeburg is quoted as slating that 

 the large weevil {Pissodes notatus) in like manner attacks the bark of young pines with its 



