TYPE TRAGEEATA. 



493 



light equal in intensity to that of the Firefly with the expendi- 

 ture of as little energy. 



The digestive tract is as a rule much more complicated 

 than in other classes of Tracheates and is generally more or 

 less twisted or contorted in the abdominal region, so that 

 usually it is longer than the body. The mouth is bounded 

 in front by a usually large upper lip or labrum, generally 

 described with the mouth-parts, but distinguished from them 

 in not representing a pair of appendages. The anterior 

 portion of the intestine, the fore-gut, is ectodermal in origin 



Fia. 337. — Stkucturb of Buttbkfly, JDanais arcJnppus (after burgess). 



a — antenna. 

 ag = accessory gland. 

 ah = accessory heart. 

 an = abdominal ganglion. 



be = bursa copulatrix. 

 c = crop. 



cc = canal uniting be and oviduct. 



ce = cerebral ganglion. 



7i = Leart. 

 I = thoracic limbs. 



mv = Malpighian tubules. 

 od = oviduct. 

 ov = ovary. 

 p?i = pharynx. 

 pi — palp. 

 s = stomach. 

 sg = salivary gland. 

 tg = thoracic ganglion. 

 I-III = thoracic segments. 

 1-9 = abdominal segments. 



as in other Tracheates and consists of a mouth-cavity 

 into which, or in its neighborhood, the ducts of one or 

 more generally well-developed salivary glands (Fig. 227, sg) 

 open. The secretion of these glands varies considerably in 

 different forms, one of the pairs present in the larvse of the 

 Butterflies and certain Moths being transformed into silk- 

 spinning glands, the silk of the Silkworm being a pi'oduct of 

 their activity. When digestive the secretion seems to have 

 a peptonizing effect as well as the power of transforming 

 starch into sugar, and is consequently of considerable diges- 

 tive importance. The mouth-cavity opens behind into an 



