LEPIDOPTERA 



577 



of a bursa copulatrix; that of the female of the milk-weed butterfly- 

 is figured on page i6o. 



Close to the junction of the thorax and abdomen there is, in the majority of 

 Lepidoptera, a pair of organs, which are known as the tympana. These are situ- 

 ated on each side near the first abdominal spiracle. Several types of these organs 

 have been described by Forbes ('i6) and by Eggers ('19), which are characteristic 

 of certain families and groups of families. 



The first type is that of the Geometridee; it appears superficially as a hollow 

 bulla located immediately below the spiracle, opening forward against the coxa of 

 the hind leg. Some Pyralidse have rudimentary tympana in the same position. 



The second type is likewise wholly on the abdomen, but it is higher on the 

 body, and its opening faces backward towards the second abdominal segment. 

 It characterizes the Thyatiridas and Drepanidse. 



The third type presents a variety of appearances. Its essential part is a mem- 

 branous disk, the tympanum proper, on the metathorax just below the root of 

 the wing. In the Dioptidse, Notodontidse, Agaristidae, and a few noctuids and 

 lithosians, the disk lies exposed or is merely sunk in a pit at the junction of the 

 thorax and abdomen. In other moths having this type of tympana the disk is 

 covered by a hood formed by the side of the first segment of the abdomen; in 

 the Arctiidag, Pericopidce, Liparidae, and the subfamily Herminiinae of the Noc- 

 tuidas, this hood lies subdorsally, wholly above the spiracle; while in the majority 

 of the Noctuidae it is lower and incloses the spiracle, in some cases (Euteliinse, 

 etc.) being supplemented by a second hood formed by the alula of the hind wing. 



The function of the tympana is probably auditory, as Eggers has described 

 chordotonal organs in connection with them in several families. 



In the Lepidoptera the metamorphosis is complete. The larvag 

 are known as caterpillars ; they vary greatly in form and appearance, 

 but are usually cy- 

 lindrical, and pro- 

 vided with from 

 ten to sixteen legs, 

 — six thoracic legs 

 and from four to 

 ten abdominal legs. 

 The thoracic legs 

 have a hard exter- 

 nal skeleton; and 

 are jointed, taper- 

 ing, and armed at 

 the end with a little 

 claw. The abdom- 

 inal legs, which are 

 shed with the last 

 larval skin, are 

 thick, fleshy, with- 

 out joints, elastic 

 or contractile, and 

 armed at the ex- 

 tremity with nu- 

 merous minute 

 hooks (Fig. 715); 



they are termed prolegs. When all five pairs are present they are 

 borne by the third, fourth, fifth, sixth, and tenth abdominal segments. 



715. — Larva of a hawk-moth. 



