6 LIFE STORIES OF AUSTRALIAN INSECTS. 



number varies in different insects. Usually the last 

 two segments are hidden from view by the over- 

 lapping of the previous segment. The horny rings 

 are joined by thinner membranes, and the body is 

 flexible. The females have an ovipositor or egg- 

 placer (Plate 3, Fig 8, a), made up of a varying 

 number of plates or parts. More usually the ovi- 

 positor is concealed within the abdomen, but In 

 many wasps is quite long and prominent. (Plate 14, 

 Fig. 3). The ovipositor can be modified to form 

 a sting or saw. (Plate 14, Figs. 4, 6, 7.) 



Internal Structure of Insects. 



Breathing. — An insect has no lungs, but the air 

 is taken into the body through a number of minute 

 pores called stigmata or spiracles. (Plate 36, Fig. 

 I, a) situated along each side of the abdomen and 

 thorax. These are the external openings to the 

 tracheae or air tubes, of which there are two main 

 branches running down each side of the body, and 

 large cross tracheae also in the abdomen and thorax, 

 and from these, innumerable branches are given off 

 so that air can be carried to every part of the body. 

 The tubes are thickened by a spiral close-wound 

 thread-like band, giving them a striated appear- 

 ance. 



Blood Vessels. — The blood system is very simple. 

 There is a very elementary heart, which is divided 

 usually into 8 chambers by a number of valves. 

 This is prolonged into a main tube or vessel which 

 carries the blood into a number of spaces in the 



