14 THE COCKROACH : 



It circulates through repeatedly-branching tracheal tubes, whose 

 lining is strengthened by a spiral coil. Air-sacs (dilated por- 

 tions of the air- tubes) occur in Insects of powerful flight. 



The generative organs are placed near the hinder end of the 

 body.* Most Insects are oviparous, f The sexes are always dis- 

 tinct ; but imperfect females (" neuters") occur in some kinds 

 of social Insects. Agamogenesis (reproduction by unfertilised 

 eggs) is not uncommon. 



Orders of Insects. 



The orders of Insects are usually denned with reference to 

 the degree of metamorphosis and the structure of the parts of 

 the mouth. Five of the orders (3, 5-8) in the table on page 9 

 undergo complete metamorphosis, and during the time of most 

 rapid change the insect is motionless. In the remaining orders 

 (1, 2, 4) there is either no metamorphosis (Thysanura), or it is 

 incomplete i.e., the insect is active in all stages of growth. 

 Among these three orders we readily distinguish the minute and 

 wingless Thysanura. Two orders remain, in which the adult is 

 commonly provided with wings ; of these, the Orthoptera have 

 biting jaws, the Hemiptera, jaws adapted for piercing and 

 sucking. 



The name of Black Beetle, often given to the Cockroach, is 

 therefore technically wrong. True Beetles have a resting or 

 chrysalis stage, and may further be recognised in the adult state 

 by the dense wing-covers, meeting along a straight line down 

 the middle of the back, and by the transversely folded wings. 

 Cockroaches have no resting stage, the wing-covers overlap, and 

 the wings fold up fan-wise. 



Further Definition of Cockroaches. 



In the large order of Orthoptera, which includes Earwigs, 

 Praying Insects, Walking Sticks, Grasshoppers, Locusts, 

 Crickets, White Ants, Day-flies, and Dragon-flies, the family of 

 Cockroaches is defined as follows : 



* Dragon-flies have the male copulatory apparatus, but not the genital aperture, 

 in the fore part of the abdomen. 



f Aphis and Cecidomyia are at times viviparous, and a viviparous Moth has been 

 observed by Fritz Miiller (Trans. Entom. Soc. Lond., 1883). 



