1921] Wheeler: Some Social Beetles 99 



and very tender young, which are scattered at random through 

 the passages, and might easily be destroyed by them in their 

 movements. If thrown into a panic, the young larvse scurry 

 away with an undulating movement of their bodies, but the older 

 larvae will frequently stop at the nearest intersecting passageway 

 to let the small fry pass, and show fight to cover their retreat. 

 When full grown the larva excavates a cell, or chamber, into 

 which it retires to undergo its transformations. The pupa cells 

 are cut parallel with the grain of the wood and generally occur 

 in groups of eight to twelve along some of the deeper passages. 

 The older portions of the galleries are blackened by the long- 

 continued formation of the food fungus. In the ambrosia of 

 Platypus composites the terminal cells are hemispherical, and 

 are borne in clusters upon branching stems." 



The habits of several genera of ambrosia beetles of the 

 family Scolytidse were investigated by Hubbard, and one of our 

 species, Xylehorus xylographus, which has an extensive circum- 

 polar distribution and is common in the wood of fruit-trees, has 

 also been studied by Eichhoff (1881) and Hopkins (1898). I 

 quote the latter's account of this insect which may serve as a 

 paradigm of the whole group : 



"The fertilized females pass the winter in their brood cham- 

 bers and emerge in the spring (April and May, near Morgan- 

 town, W. Va.). They are then attracted to sickly, dying or 

 felled trees, in the living or moist dead wood of which they 

 prefer to excavate their brood galleries. A crevice or opening in 

 the bark, such as may be made by other insects, or, as I have 

 observed, those made by the yellow-bellied woodpecker, but more 

 commonly the edge of a wound, in a dead place on a living tree, 

 is selected as a favorite point of attack. Here a female will 

 commence the excavation of a mine, and after she has penetrated 

 the wood a short distance, another female (as I have observed) 

 will come to her assistance, one working at the excavation, while 

 the other guards the entrance and assists in expelling the borings. 

 The primary or main gallery is usually extended into the heart- 

 wood before eggs are deposited. When the primary gallery is 

 completed (according to Hubbard) a bed is provided on the 

 sides of the gallery for the propagation of the special species or 

 variety of ambrosia fungus which is to furnish food for the 



