8 NATURAL SCIENCE [July 



A Study in Parasitism 



Bulletin No. 5 (Technical Series) of the same department con- 

 tains an exhaustive account by Mr L. D. Howard of the insect- 

 enemies of the white-marked tussock moth (Orgyia leucostigma). 

 The hairy caterpillars of this moth have become, during recent 

 years, highly injurious to foliage trees in the cities of the northern 

 States. It appears that the vast multiplication of these caterpillars 

 began with the introduction of the European sparrow. This bird 

 has well-nigh exterminated other caterpillars which used to compete 

 with the ' tussocks ' for a livelihood on the trees ; it will not touch 

 the hair-protected ' tussocks ' itself, but it has largely driven out 

 the native birds which used to feed on them. Fortunately, how- 

 ever, an army of insect-parasites keep the caterpillars from increas- 

 ing beyond all bounds. Fifteen species of Hymenoptera and six of 

 Diptera are described as laying their eggs in either larva or pupa of 

 the Orgyia. These parasites are in their turn subject to attack by 

 thirteen species of minute Hymenoptera. An alarming increase of 

 the ' tussocks ' at Washington during the summer of 1895 was 

 accompanied by a corresponding increase in the parasites, so that 

 the vast majority of the caterpillars were destroyed. The next year, 

 however, owing to the work of the secondary parasites, the ' tussock ' 

 recovered its ground to a considerable extent. Two species of tiny 

 hymenoptera were proved to be ' tertiary ' parasites, their grubs 

 devouring those of the secondaries. Finally, eleven species of 

 Diptera act as scavengers, their grubs feeding on the dead pupae and 

 cocoon-masses of the contending insect -armies. 



A Study in Commensalism 



It is cheering to turn from this summary of conflict and slaughter 

 to another side of insect-life lately investigated by Mr C. Janet. 

 In a little work of 6 2 pages (" Sur le Lasius mixtus, YAntennophorus 

 uhlmanni, &c. " : Limoges, 1897) he describes the relations which 

 subsist between social ants (Lasias) and certain gamasid mites and 

 other arthropods which are found in their nests. The mites of the 

 species Antennophorus uhlmanni are carried about by the worker 

 ants ; in their penultimate and adult stages they seem unable to 

 make their own way about the nest, and wait feeling with their long 

 front legs for an ant to which they can attach themselves. An ant 

 normally carries three of the mites — one under the head and one on 

 either side of the abdomen ; these positions are the least incon- 

 venient possible to the ant. The mites feed entirely on liquid dis- 

 gorged by their hosts. An individual under an ant's head naturally 



