4 1 - AXTS. 



ceous animals, for in both cases the victimized species exists at the 

 present time only because it has great reproductive powers or a margin 

 of redundant vitality which can be exploited by its enemies and para- 

 sites ; and the survival of these enemies and parasites themselves in 

 turn depends on their refraining from overstepping this margin. In 

 the case of saiu/itinca the enormous reproductive powers of the species 

 must more than compensate for the destruction of colonies by the 

 Lomechusa. 



Ectoparasites. With the Lomechusini we may close our account 

 of the true guests, although these include also several other interesting 



forms, like Loincchon among the Silphidae, 

 Amorphocephalus among the lirenthidse 

 and Pogonoxenus among the Tenebrionidae. 

 Turning to the parasites proper, we find it 

 impossible to draw a hard and fast line 

 between symphiles and ectoparasites, owing 

 to the existence of such intermediate forms 

 as Thorictus, Antennophorus and Orascina. 

 FIG. 248. Thorictus fo- The group of ectoparasites as a whole is 



reli in its normal position, , ' , 



attached by means of its jaws a heterogeneous assemblage of mites, 

 to the antennal scape of Myr- Hymenoptera, Diptera and Coleoptera. 



mecocystus mesalocola. (Or- ~ , , 



1 he Loleoptera, however, are represented 



only by certain species of Thorictus. 



Some of the ectoparasites, of which the Phorid flies of the genus 

 Metopina and the Gamasid mites of the genus Antennophorus may be 

 taken as interesting examples, are hardly more than commensals. In 

 two papers (1901^, 19070) I have described the singular habits of Meto- 

 pina pachycondylce, which lives in Pachycondyla harpa.r colonies in 

 Texas. Its small larva clings to the necks of the ant-larva by means of 

 a sucker-like posterior end and encircles its host like a collar ( Fig. 243 ). 

 Whenever the ant-larva is fed by the workers with pieces of insect 

 placed on its trough-like ventral surface, within reach of its mouth- 

 parts, the larval Metopina uncoils its body and partakes of the feast; 

 and when the ant-larva spins its cocoon it also encloses the Metopina 

 larva within the silken web. The commensal, however, moves to the 

 caudal end of its host and forms a small, flattened puparium which is 

 applied to the wall of the cocoon. This is obviously an adaptation for 

 preventing injury from the jaws of the worker ants when the cocoon 

 is being opened and the callow extracted from its anterior end. The 

 ant hatches before the Metopina and the empty cocoon with the pupar- 

 ium concealed in its posterior pole is carried to the refuse heap. There 

 the fly emerges and escapes from the cocoon by the opening through 



