HOW INSECTS BREATHE. 



19 



strengthened by hollow rods called "veins;" their branches 

 being the " venules." There are in the wings of most insects 

 six main veins i.e., the costal, the subcostal, 

 median, submediau, internal, and anal. They 

 are hollow and usually contain an air-tube, 

 and a nerve often accompanies the trachea in 

 the principal veins. The arterial blood from 

 the heart (as seen in the cockroach by Mose- 

 ley) flows directly into the costal, subcostal, 

 median, and submedian veins; here it is in 

 part aerated, and returns to the heart from 

 the hinder edge of the wings through the 

 hinder smaller branches and the main trunks 

 of the internal and anal veins. 80 that the 

 wings of insects act as lungs as well as organs 

 of flight. For the latter purpose, the prin- 

 cipal veins are situated near the front edge 

 of the wing, called the costa, and thus the 

 wing is strengthened where the most strain 

 comes during the beating of the air in flight. 



The wing of an insect in making the strokes during flight 

 describes a figure 8 in the air. A fly's wing makes 330 

 revolutions in a second, executing therefore 660 simple 

 oscillations. 



How Insects Breathe. Insects breathe by means of a 

 complicated system of air-tubes ramifying throughout the 

 body, the air entering through a row of spiracles, or breath- 

 ing-holes (xtir/tn<tt(i), in the sides of the body. There are 

 in locusts two pairs of thoracic and eight pairs of abdominal 

 spiracles. The first thoracic pair (Fig. 2) is situated on 

 the membrane connecting the prothorax and meso thorax, 

 and is covered by the hinder edge of the pronotum (usually 

 called prothorax). The second spiracle is situated on the 

 posterior edge of the mesothorax. There are eight abdominal 

 spiracles, the first one situated just in front of the auditory 

 sac or tympanum, and the remaining seven are small open- 

 ings along the side of the abdomen (Fig. 2). From these 



FIG. 16. Foot- 

 tracks of Necro- 

 phorus respillo. 

 Natural size. 

 After Graber. 



