296 THE SEA SHORE 



Insects are developed from eggs, but in their young state they 

 are segmented larvae, with strong jaws, antennae, simple eyes, and 

 usually three pairs of legs attached to the first three segments next 

 to the head. 



As regards internal structure, we need only mention here that 

 the body is traversed by numerous branching tubes (tracheez) that 



open at the exterior and con- 

 stitute the respiratory appa- 

 ratus ; that the insect is 

 provided with a contractile, 

 tubular heart by means of 

 which the blood is propelled 

 through a system of blood- 



Fia. 221. TRACHEA" OF AN INSECT, vessels; that the nervous 

 MAGNIFIED system consists of a chain of 



ganglia, connected by a nerve 



cord, sending nerve filaments to all parts of the body ; and that 

 the digestive tube is often a complicated structure, especially in 

 the case of those insects that feed on herbivorous matter. 



The above outline will be sufficient to show that insects are not 

 very unlike the crustaceans in their general characteristics ; and, 

 indeed, when we examine certain forms, noting the distinct segmen- 

 tation of the body, the hardened exo-skeleton of chitinous material, 

 and the unhardened skin between the segments to admit of freedom 

 of movement, we see a striking resemblance in external appearance 

 to some of the typical crustaceans. 



Insects are divided into several orders, and some of these are 

 fairly well represented on the sea coasts, though it must be under- 

 stood that but few species are strictly aquatic and marine in their 

 habits. Fresh-water pools and streams teem with insect life, and 

 quite a large number of the insects that live in these situations -are 

 peculiarly adapted for a life of submersion, their general form being 

 often such as to allow of rapid progress through the water, their 

 appendages modified into admirable swimming organs, and, in many 

 cases, their breathing apparatus adapted for the direct absorption of 

 oxygen dissolved in the water. 



However, one would hardly expect to find similar forms of life 

 abundant in the water that washes our shores, the disturbing action 

 of the waves, even in calm weather, being more than such fragile 

 creatures could withstand. And this is really the case, for there 

 are but few insects that may be described as marine in the strictest 



