574 



INSECTA. 



ovaries are not connected with any special bursa copulatrix, but 

 have three receptacula seminis in connection with the vagina 

 (fig. 449), and often end with a retractile ovipositor. 



There is rarely a striking difference between the two sexes. The 

 males have as a rule larger eyes, which in some cases meet each other 

 in the middle line ; their abdomen also is frequently differently 

 shaped to that of the female, and in exceptional cases the colouring- 

 is different (Bibio), The mouth-parts, too, may differ ; for example, 

 the male gad-flies (TabanidcK) are without the knife-shaped mandi- 

 bles, which form the principal part of the female armature. The 

 males of the Culicidce also are without the piercing weapons, and 

 have multiarticulate hairy antenna?, while the antennae of the female 

 are filiform, and are composed of fewer joints. 



The metamorphosis is complete, and 

 the larvae, which are usually apodal, 

 have either a clearly separate head 

 with antenna? and ocelli (most Netno- 

 cera), or a short, usually retracted, 

 cephalic region, without antenna? or 

 eyes (at most with an X-shaped pig- 

 ment spot), with quite rudimentary 

 mouth parts, sometimes with tAvo oral 

 hooks, serving for attachment. 



In the first case the larva? have 

 masticating mouth-parts and feed on 

 other animals ; in the latter case they 

 are known as maggots and suck up 

 fluids or semi-liquid substances. After 

 several moults the larva? either change 

 within the hardened larval skin to pupa? (P. coarctata), or casting 

 the larval skin are transformed into moving pupa? (P. obtecta), which 

 often swim freely in water, and may be provided with tracheal gills. 

 The differences which the development of the winged insect from 

 the larval organism presents in the two groups have been already 

 mentioned (p. 550). 



Many Diptera when flying give rise to buzzing sounds. This is 

 caused by the vibrations of various parts of the body ; partly of 

 the wings and partly of the segments of the abdomen, with 

 participation of the voice apparatus on the four stigmata of the 

 thorax. Here, beneath the margins of the stigmata, the tracheal 

 trunk forms a vesicle with two delicately folded leaflets, which 



(t 



FIG. 476.lleIop}i<iifiis acinus, b, 

 eqii'nm (after Packard). 



