NO. 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 3I 



plates are then braced against the sternum. The small intersegmen- 

 tal sternites are known as the spinastema because each usually sup- 

 ports a pair of internal processes for muscle attachments. There is, 

 of course, much diversity in the relative size of the sternal parts in 

 different insects, and a large part of the venter in each segment may 

 be membranous. If the sternal sclerotization extends to the base 

 of the leg on each side it forms a ventral articular process for the 

 coxa. 



While the above description applies to the thorax of most winged 

 insects, there are structure and function variations in the different 

 orders. In the honey bee, for example, a line of flexion in the 

 mesonotum cuts across the posterior part of the scutum and scutel- 

 lum between the bases of the wings. In two-winged insects the 

 metathorax is generally much reduced in size but retains the funda- 

 mental thoracic structure, showing that the 4-winged condition is 

 primitive for these groups. 



Spiracle: The breathing apertures should be called spiracles 

 from the Latin word spiro, to breathe, and spiraculum, a breathing 

 hole. The term stigmata formerly given to them means "spots," and 

 probably reflects the ignorance of the early entomologists regarding 

 their function. A "stigma" was also a brand with which slaves were 

 marked, and hence a blemish. The word should be discontinued as 

 an entomological term. 



A spiracle is more than simply a hole into a trachea ; it is usually 

 a depression of the cuticle into which the trachea opens, forming 

 thus a spiracular atrium. Most spiracles have a special closing ap- 

 paratus which may be the outer lips of the atrium but more com- 

 monly is a valvelike structure at the opening of the trachea operated 

 by muscles. 



Leg: The Latin word for leg is cms, cruris, but this word and 

 its Greek equivalent skelos have not been adopted into anatomical 

 terminology for the legs of an animal though we have the crura 

 cerebri of the brain and crural nerves of the legs. On the other 

 hand, the Latin pes, pedis, and the Greek pons, podos, each strictly 

 meaning the foot, have become the basis of most leg names, as in 

 biped, centipede, milliped, arthropod, diplopod, hexapod, etc. 



In insect anatomy the word leg is used in a functional sense rather 

 than a morphological one, since it is applied to the thoracic legs and 

 to the abdominal prolegs of larvae though the two sets of organs 

 have no homology. The thoracic legs represent the embryonic leg 



