210 



INTRODUCTION TO ZOOLOGY 



this way the complete image, formed at the back of the eye 

 on the retina, will be a mosaic of a number of partial images 

 each formed by a different lens. In whatever way the image 

 may be produced, such a projecting compound eye is evidently 

 a very effective organ for warning its possessor of the 

 approach of any object, for an insect is very difficult to 

 surprise. 



Simple eyes, or ocelli, are also very frequently found in adult 

 insects, and in the larval stage they are generally the only 

 eyes present. They are quite simple in structure, consisting 

 sometimes merely of a mass of pigment lying over a nerve 

 end, though most frequently over the pigment patch is a 

 small transparent lens serving to focus the light on to it. 



All adult insects breathe ordinary air, though in 

 ofTusects" ^^ larval state, if aquatic, they may be able to 

 absorb, through special gills, the air dissolved 

 in the water. Adult insects breathe, not through their 

 mouths, as Vertebrates do, but through a number of little 

 special openings or spiracles, several of which are usually to 

 be found on each side of the body. These spiracles lead 

 into a complex system of air-tubes or tracheae (Fig. 137) 

 which carry the air to all parts of the body, forming a 

 "egular network of minute tubes round 

 ihe internal organs, and thus coming into 

 ;lose contact with the blood-vessels that 

 .ilao surround the organs, and bringing 

 about the aeration of the blood which is 

 the function of respiration. In insects 

 there is no need for any specially localised 

 respiratory organs where blood and air 

 meet, such as are present in Molluscs and 

 Crustacea, for the air is carried direct 

 by the tracheae to the blood-vessels all 

 over the body. 



The wall of each trachea is strengthened, 

 on its inner side, by a chitinous spiral 

 thread, 'which serves, to keep the cavity 



of a Bee. freely along (lig. 142). Ihe renewal of 



the air in these air-tubes seems frequently 



to be brought about by rhythmic movements of the abdomen 



