492 INVERTEBRATE MORPHOLOGY. 



A dermal muscular system does not exist, but complicated 

 and well-developed muscles are present for the movement of 

 the various parts of the body, those occurring in the thorax 

 being especially well developed and serving for the movement 

 of the limbs and wings. As in other Tracheates the coelom 

 is lacuuar, and the heart lies in a pericardial sinus below the 

 dorsal surface of the body, alar muscles extending from it to 

 the walls of the body and partly dividing the sinus into a dor- 

 sal and a ventral chamber. In the Thysanura the heart 

 extends from the posterior thoracic region throughout the 

 greater part of the abdomen, and consists of nine chambers 

 separated by valves and each provided with a pair of ostia and 

 a pair of alar muscles. In the majority of forms (Fig. 227, li), 

 however, the heart is entirely confined to the abdominal 

 region, and the number of chambers, though frequently as 

 high as eight, may be greatly reduced. An aorta extends 

 forwards from the anterior chamber into the head, in the 

 Butterflies (Fig. 227) dilating in the thorax to form a second- 

 ary heart (all), and sends off branches which quickly empty 

 into the lacunar spaces. 



The greater portion of the abdomen is occupied by a 

 peculiar tissue, termed the fat-body, in which the various 

 organs are more or less imbedded, and which receives its 

 name from the fact that its cells contain globules of fatty 

 matter, and in the adult insect usually also concretions of uric 

 acid. It is not necessarily confined to the abdomen, but may 

 extend into the thorax or even into the head. In certain 

 Beetles the Fireflies (Lampyridse) and Pyrophorus of the 

 West Indies certain regions of the body, especially the 

 abdomen, and, in Pyrophorus, two spots upon the thorax, give 

 out under certain conditions, apparently under control of the 

 animal, a very bright light, usually spoken of as a phosphores- 

 cence. The tissue which produces the light is the fat-body, 

 or special portions of it abundantly supplied by tracheae, and 

 the process seems to be one of oxidation of phosphorus- 

 containing substances. The exact nature of the phenomenon 

 is but poorly understood at present, and it is not possible by 

 any means at 'our disposal to produce in the laboratory a 



