INDIRECT BENEFITS DERIVED FROM INSECTS. 229 



Generally speaking, parasitic larvae do not attack insects 

 in their perfect state ; but to this rule there are several ex- 

 ceptions. M. Dufour found in a beetle (Cassida viridis) a 

 parasitic larva, from which he bred a fly of the genus Tachina 

 Meig. (^Cassid<Emyia Macq.) ; and also in a field-bug {Pen- 

 tatoma grisea), from which proceeded another fly (^Ocyptera 

 bicolorY'i and Latreille, Dufour, and other entomologists 

 have confirmed the discovery of Baumhauer, that the larvae 

 of flies of the genus Conops live in humble bees, which M. Ko- 

 bineau-Desvoidy has seen pursued by them, apparently to 

 deposit their eggs on them. ^ The larva) of a beetle (^Simbius 

 Blattarurn) is parasitic in the bodies of Blatta americana on 

 board of ships, and M. Audouin found Coccinella 11 -punctata, 

 to be subject to the parasitic attack of Microctonus terminalis 

 Wesmael, and Encrytus Jlaminius Dalman.^ 



The order also of Strepsipjtera appears to be wholly para- 

 sitic ; but these extraordinary animals are found only upon 

 Hymenoptera in their perfect state, and do not appear to de- 

 stroy the insects upon which they prey, but probably prevent 

 their breeding. The species at present known are formed 

 into four genera, Xenos Rossi ; Stylops Kirby ; Elenchus 

 Curtis ; and Halictophagus Dale. The first is found in dif- 

 ferent species of wasps ( Vespa, Polistes, Odynerus, and also 

 of Sphex); the second in the genus separated from Melitta K. 

 under the name of Andrcena, in upwards of fourteen species 

 of which Mr. Pickering has found them ; the third in Polistes? ; 

 and the fourth in Halictus (^Melitta K.) ; but it is probable, 

 from the fact of M. L. Dufour's having also found a larva of 

 one of these insects between the abdominal segments of Am- 

 mophila Sabulosa, that many other hymenopterous insects will 

 be found to be infested with them. ^ 



1 Macquart, Dipteres, ii. 69. 



2 Ibid. ii. 23. Westwood, Mod. Class, of Ins. ii. 561. 



3 Westwood, Mod. Class, i. 295. 397. 



■t Kirby, Man. Ap. ^n^. ii. 1 10. 113. and in Linn. Trans. x\. 86. West- 

 wood's Mod. Class, of Ins. ii. 288 — 305., to which last the reader is referred for 

 a full and very interesting account of the facts hitherto recorded respecting these 

 remarkable insects, and references to the various works in which they occvir. My 

 friend G. H. K. Thwaites, Esq. has had the singular good fortune, which has 

 perhaps occurred to no other entomologist, of seeing on the wing in May, 1838, 

 not merely a single stylops or two, but a small swarm of at least twenty, and in 

 as singular a situation, the garden of his residence, situated in the suburbs of the 



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