252 Arthropoda. 



covered with water^ so that their inhabitants are excluded from the 

 air for hours together;* the Halobatidse are the only actually marine 

 forms, and these lead a life on the open sea like that of their near 

 allies, the H^'^drometridee, in freshwater.t Various Insects (Pediculidse, 

 Mallophagidse, Pulicidae, etc.), live as imagines, or, during the whole 

 of their life, as parasites on various Vertebrates ; others are parasitic 

 only as 1 a r v se upon, or in, various other animals, whilst they lead 

 a free existence as perfect-insects. 



The Insecta are richer in species than any other class of animals. 

 According to one reckoning they make up four-fifths of all species ; of 

 Insects again, one half are Beetles. 



Hemimetatola ( 2f*^°P*^f Biting mouth-parts. 



t Khynchota Sucking „ „ 



Nein-optera -j 



Ooleoptera > Biting „ „ 



Holometabola •{ Hymenoptera J 



Lepidoptera | ^^^^^ 



Diptera ) 



Order 1. Orthoptera. 



The Orthoptera are hemimetabolous Insects with biting 

 mouth-parts. The labium shows more clearly than in other 

 forms that it has arisen by the fusion of a pair of jaws, the 

 individual portions of which are usually easily recognisable. The 

 wings are generally closely veined, but in other respects differ 

 considerably. Frequently the number of abdominal segments is. 

 large ; the abdomen is usually furnished with two shorter or 

 longer jointed or unjointed, anal cerci. This order includes very 

 various forms ; of the types given below, numbers one to six have 

 the front pair of wings modified to form leathery elytra, whilst in 

 the rest all the wings are similar. 



1. Grasshoppers (genus ^cricimm-, and others). The limhs of the last 

 pair are long, springing legs with thickened tibise. The front wings form long,, 

 nan-ow, somewhat thickened elytra, below which lie the broad hind wings folded 

 up like fans. The prothorax is large, the antennae short and filiform (at most 

 only twenty-four joints), auditory appai'atus (see p. 238) on the first abdominal 

 segment. The males make a noise by nibbing a dentate ridge of the femur of 

 the last leg against the elytron. The females have no ovipositor. Various small 

 species are often met with in great numbers in the fields. Certain species (some 

 large, others small) are common in wai-m ooimtries as " migratory Locusts " 

 {e.g., Acridium migratoriwm) ; these, after increasing enormously in some locality, 

 migrate in incredibly large numbers, utterly destroying all vegetation in the 

 regions through which they pass. 



* This is also the case with some of the Arachnida (Mites and Pseudosoorpions) 

 and Seolopendras. 



t The Lice parasitic on Seals are also marine. 



