88 ARTHROPOD A. 



seen in some flies (Diptera), or may remain separate, as in many 

 Orthoptera {Cockroaches). 



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 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 acariasio diseases in stock ; whilst even 

 the aquatic Crustacea have amongst their vast concourse of 

 species some, such as the Wood-louse, which have taken to 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 Coccidse produce not only cochineal but also 

 sealing-wax, shell-lac, and other products. The Silkworm and 

 the Bee alone make up for the many deficiencies of insect life ; 

 and we are not sure that all harm accrues from such a destructive 

 insect as the Wireworm, for one and aU teach us to be more 

 careful in regard to cultivation — that is, cleanliness and im- 

 provement. The bad farmer is he who suifers most from these 

 enemies. 



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 as we have to deal more fully 

 with the latter group, we will take one of them to point out the 

 peculiarities and general structure of the Jointed-limbed Inver- 

 tebrates. 



