ARTHROPOD A. 99 



flies [Dipfera), or may remain separate, as in many Orthoptera 

 (CocJiroaehes). 



There is great variation in form and in habits of these various 

 jointed-limbed animals, which can be enumerated as each group 

 is taken. That they are of much economic importance to the 

 farmer, gardener, and stock-keeper is needless to state. Insects 

 and mites are accountable for endless loss to the farmer, fruit- 

 grower, and gardener. Mites {Acarina) not only devastate our 

 fruit and hops, but are the cause of such wasting diseases as 

 " scab," mange, and various other acariasic diseases in stock ; 

 whilst even the aquatic Crustacea have amongst their vast con- 

 course of species some, such as the Wood-louse and Crayfish, 

 which have a terrestrial life and have become injurious to 

 mankind. We must not be led away, nevertheless, with the 

 idea that all Arthropods are our enemies, and so ruthlessly 

 destroy them : many are of infinite help to us as agents for 

 keeping down and destroying those that do us harm, whilst 

 such insignificant creatures as the Coccidfe produce not only 

 cochineal but also sealing-wax, shell-lac, aJid other products. 

 The Silkworm and the Beo alone make up for the many 

 deficiencies of insect life. 



The Arthropods are divided into four sections, namely — 



1. Crustacea (Crabs, Lobsters, Shrimps, &c.) 



2. Araneida (Spiders, Mites, Scorpions, &c.) 



3. Myriapoda (Centipedes, &c.) 



4. Hexapoda (Insects). 



The Crustacea and the Hexapoda have the Arthropod char- 

 acters most clearly defined, and we will take one of the latter 

 group to point out the peculiarities and general structure of the 

 Jointed-limbed Invertebrates. 



For this purpose we cannot choose a better example than the 

 Cockroach {P&'iplaneta americana), which can be easily ob- 

 tained, as our type. This species, although not so universal 

 as the common Cockroach, the so-called Black-Beetle of our 

 kitchens (Blatta orientalis), offers better scope for dissection. 



