148 Descriptions of British Diptera. 
the power of again replenishing its little reservoir by forcing air into 
it from the tracheae. 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 obleng 
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. 35, larva. La- 
treille, Gen. Crust, iv. 274. Panzer, Fauna Germ. viii. 24; Geoffrey, 
Ins. ii. pi. 17. fig. 4; De Geer, Ins. vi. 151, 1 Swammerdam, Book of 
Nat. pi. 39 to 42 Stratiomys chamaeleon, Meigen, iii. 124 ; Stephens, fyc. 
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 life, 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 antennae, the eyes, and the mouth. 
