INDIRECT BENEFITS DERIVED FROM INSECTS. 155 



Generally speaking, parasitic larvae do not attack insects in their perfect 

 state ; but to this rule there are several exceptions. M. Dufour found in 

 a beetle (Cassida viridis) a parasitic larva, from which he bred a fly of the 

 genus Tachina Meig. (Cassid&myia Macq.) ; and also in a field-bug (Pen- 

 tatoma grisea), from which proceeded another fly (Ocyptera bicolor] 1 ; 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. Robineau-Desvoidy has seen pursued by them, apparently 

 to deposit their eggs on them. 2 The larvae of a beetle (Simbius Blattaruni) 

 is parasitic in the bodies of Blatta Americana on board of ships, and 

 M. Audouin found Cocdnella 17-punctaia, to be subject to the parasitic 

 attack of Microctonus terminalis Wesmael, and Encrytus flaminius Dalman. 3 



The order also of Strepsiptera appears to be wholly parasitic ; but these 

 extraordinary animals are found only upon Hymenoptera in their perfect 

 state, and do not appear to destroy the insects upon which they prey, but 

 probably prevent their breeding. The species at | resent known are formed 

 into four genera, Xenos Rossi , Stylops Kirby ; Elenchus Curtis ; and 

 Halictophagus Dale. The first is found in different 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 Ammophila Sabulosa, that many 

 other hymenopterous insects will be found to be infested with them. 4 



plants, when M. Audouin examined them with him near Pisa in 1835, were covered 

 with eggs, which the former recognised as altogether similar to those of Sitaris hu- 

 meralis, with which he was well acquainted. As the species of Sitaris are known to 

 be found in the nests of different Hymenoptera, and particularly in those of a wild 

 bee (Anthophora) on the larvae of which their Iarva3 are probably parasitic, the ques- 

 tion occurs, with what view these eggs were placed on the rosemary ? The most 

 plausible supposition perhaps would seem to be that after the eggs are hatched the 

 larvae attach themselves, like the supposed larvae of Meloe (Pediculus Melittae K.) to 

 which they are related, to the Anthophorce, frequenting the rosemary for honey, and 

 are thus conveyed into their cells ; but nothing certain can be inferred on this head 

 till the contradictory statements as to these last-named larvae are cleared up ; and 

 it seems as yet almost equally doubtful (as it is also in the case of the other para- 

 sitic coleopterous genera Horia, Ripiphorus, and Zonitis) whether the larvae are para- 

 sitic on the larvae of the insects in whose cells they are found, or on their stored-up 

 food. 



7 Westwood, Mod. Class, of Ins. i. 332. 



1 Macquart, Dipteres, ii. 



2 Ibid. ii. 23. We 



^estwood, Mod. Class, of Ins. ii. 561. 



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



4 Kirby, Mon. Ap. Ang. ii. 110. 113. and in Linn. Trans, xi. 86. Westwood's 

 Mod. Class, of Ins. ii. 288305, 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 occur. My friend G. H. 

 K. Thwaites, Esq., has had the singular good fortune, which has perhaps oc- 

 curred 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 populous city 

 of Bristol. This was most probably owing to the circumstance of the garden hav- 

 ing had brought into it a quantity of fresh earth, which apparently had been dug 



