272 CLASSIFICATION OF ANIMALS 



several salivary glands are connected. The oesophagus is often 

 enlarged to form a crop. In several orders there is also a 

 gizzard between the crop and the stomach. The stomach is 

 the true digestive portion of the alimentary canal. It is larger 

 in diameter than oesophagus or intestine and usually has sack- 

 like or tubular glands opening into its anterior end. Following 

 the stomach is first a narrow small intestine and then a wider 

 large intestine. At the junction of stomach and small intestine 

 there are a number of long and very slender tubes, known as 

 Malpighian tubules. These are the excretory organs of the 

 insect. The respiratory system consists of a greatly branched 

 system of tracheae. These open on the surface at the side of the 

 abdomen and thorax. The tracheal capillaries extend to all 

 parts of the body. The heart is a long contractile vessel lying 

 on the dorsal side of the abdomen. The blood enters it through 

 eight pairs of ostia segmentally arranged. A vessel leads 

 forward from the heart into the head. The blood circulates 

 through the body cavity. 



580. There are usually a pair of highly developed compound 

 eyes and sometimes two or three ocelli. Other special sense 

 organs are the tactile hairs on the antennae, the olfactory cones 

 and pits of the palpi and taste cells of the mouth cavity. The 

 "brain" is large and complex in structure. The ventral gang- 

 lionic chain may in reality be a chain of as many as twelve 

 ganglia, but various stages of concentration occur even to the 

 fusion of all into one mass. 



581. The gonads open by a pair of ducts at the posterior 

 end of the abdomen. The sexes are separate and usually di- 

 morphic. In some orders polymorphism is not uncommon. 

 The eggs, in many cases, develop without fertilization (par- 

 thenogenesis) . In most orders there is a marked metamorphism. 



582. Order 1. — The Orthoptera are insects with biting mouth parts, two 

 pairs of wings which are unhke, and development by an incomplete meta- 

 morphosis. The order includes, earwigs, cockroaches, the praying- 



