148 Descriptions of British Diptera. 



the power of again replenishing its little reservoir by forcing air into 

 it from the trachea?. Its motions in the water are very slow, and it 

 advances by bending its body in different directions. Swammerdam 

 supposed that it fed on clay and soft earth, substances which he 

 sometimes found on dissection lodged in the stomach; but it is much 

 more probable that its food consists of the minute infusoria which 

 abound in the stagnant waters which it usually frequents. * It is 

 furnished with short ciliated organs on each side of the head, which 

 are frequently kept in a state of rapid vibration, by which slight cur- 

 rents are produced in the direction of the mouth, — a circumstance 

 which would tend, on the supposition just made, to bring its food 

 completely within its reach. When about to change into nymphs, 

 the skin of these larvae becomes stiff and rigid, and in that state 

 forms an envelope for the pupa, which is contracted into an oblong 

 figure, and occupies but a small space towards the widest end of its 

 capacious tube. The fly is soon matured, sometimes in six or seven 

 days, and makes its escape by forming a rent in the second segment. 

 It is unnecessary to advert at any length to the opinion entertain- 

 ed by Professor Knoch, and countenanced by Meigen, respecting the 

 larvae of the Stratiomydes, namely, that they are of a parasitical na- 

 ture, and live in the interior of the aquatic worms above described, 

 which according to them have long been erroneously regarded as 

 true dipterous larvae. The observations on which this view profes- 

 ses to be founded are quite inconclusive, and it is in itself so highly 

 improbable, that it is surprising how it could be advanced in oppo- 

 sition to the well authenticated statements of the older naturalists. 



1. Stratiomys chameleon. fs.J 

 Musca chamseleon, Linn. ; Donovan, i. pi. 31, fig. 1st, pi. 85, larva. La- 

 treille, Gen. Crust, iv. 274. Panzer, Fauna Germ. viii. 24 ; Geoffroy, 



Ins. ii. pi. 17. fig. 4; De Geer, Ins. vi. 151, 1 Swammerdam, Book of 



Nat. pi. 39 to 42 — Stratiomys chamaaleon, Meigen, iii. 124 ; Stephens, Sfc. 



Head and antennae black, the former with the hypostomet black in 

 * Swammerdam affirms that these insects are found in salt water as well as 

 fresh ; but as this statement has not been confirmed by subsequent observation, 

 and is contrary to what takes place in almost every other instance, it is probable 

 that it was either made inadvertently, or that the larvae noticed in such situa- 

 tions by that illustrious physiologist had been transported thither accidentally, 

 which might easily happen by means of running water. Once conveyed thither 

 they might survive for a length of time, for they can live upwards of twenty-four 

 hours in spirits of wine and vinegar. So tenacious, indeed, are they of bfe, that 

 after immersing them in several fluids without effect, Swammerdam's patience 

 became exhausted, and he proceeded to dissect them alive. 



f This term, which it is convenient to retain, is employed by Meigen to desig- 

 nate the region of the head lying between the antenna?, the eyes, and the mouth. 



