A REVIEW WITH HYPOTHETICAL CONCLUSIONS. 105 



It certainly is— both from a logical basis, and by analogy with other insects. 

 Though tempted, the writer believes it better not, at this time, to review some of 

 the strong confirmatory evidence of specialised migratory flight of solitary 

 insects, such as that supplied by the amazing rate of dispersion often 

 characteristic of species newly introduced into a favourable faunistic region ; 

 of swarms of lowland insects entrapped on mountains ; of inland insects on the 

 seashore ; of southern insects in northern localities ; of the curious behaviour of 

 many species at light, etc. He does, however, desire to illustrate the point, 

 enlarging somewhat upon the migratory habits of a certain species of Scolytid, 

 Dendroctonus frontalis. 



This is a gregarious, but not a social insect, though it exhibits a slight 

 tendency to become so. It attacks living and perfectly healthy, as well as injured 

 and dying, pines in the south-eastern portion of the United States. The beetles 

 are attracted to a tree that has been injured (as by lightning), usually in the 

 midst of a pine grove or forest. They swarm over this tree, and, mining in the 

 bark, deposit their eggs. No matter how many may be drawn to the spot, no 

 more attack this one tree than can breed in it successfully. If there are too 

 many, the others gather on healthy trees immediately adjacent. They do not 

 scatter on many trees, for if they did, the pitch running into their galleries in the 

 bark would kill or drive them out. Instead, they concentrate on as many trees 

 as they can quickly kill, and thus soon stop the flow of resin. In about a month 

 or six weeks the young generation begins to issue. Usually they attack the 

 perfectly healthy trees standing next outside of those already killed, and all the 

 trees are thus killed over an irregular area about the original centre of attraction. 

 The beetle usually increases in numbers very rapidly with each generation, two 

 generations nearly always, and frequently three, being passed in the manner 

 described. A score or hundred or several thousand trees may be killed. With 

 very rare exceptions the young beetles do not remain in the vicinity after the 

 second or third generations to attack the adjacent healthy pines. They migrate 

 instead ; with a unanimity which, like the idea of rats leaving a doomed ship, it 

 is uncanny to contemplate, they fly the spot. There may be hundreds of 

 thousands, but they go to the last one. This migration takes place notwith- 

 standing that the locality is proven favourable for their rapid reproduction, and 

 though an abundance of adjacent trees remain open to attack. The issuing 

 beetles go deliberately, and many of them go to their own destruction, because a 

 few together cannot attack living trees successfully. They must attack in force, 

 and when they fly they seem to scatter, since it requires a special centre of 

 attraction to draw them together again in numbers sufficient to overcome the 

 natural resistance of healthy trees. They cannot even breed to advantage 

 in prostrate trees and logs, though they often attempt to do so to the destruction 

 of their brood. 



Nothing that is proposed in the way of a migratory instinct that shall lead 

 Glossina to desert favourable breeding spots for less favourable is so striking an 

 example of a specialised instinct (developed for the good of the species as a 

 whole) as is this migratory habit of Dendroctonus frontalis. For if the species 

 did not migrate in the manner described, the result would be that the entire 

 countryside would be denuded of pine and the insect very likely exterminated as 



