ENTOMOLOGY 



arthropods have descended from annelid-like ancestors. Annelids, 

 however, as contrasted with arthropods, have segments that are essen- 

 tially alike, have no external skeleton and never have paired limbs that 

 are jointed. 



Classes of Arthropoda. Excepting the king-crab, trilobites and a 



few other aberrant forms of uncertain 

 position, the members of the series, or 

 phylum, Arthropoda fall into six dis- 

 tinct classes, namely, Crustacea, Arach- 

 nida, Malacopoda, Diplopoda, Chi- 

 lopoda and Insecta. These classes are 

 characterized as follows: 



Crustacea. Aquatic, as a rule. 

 Head and thorax often united into a 

 cephalothorax. Numerous paired 

 appendages, typically biramous (Y- 

 shaped); abdominal limbs often pres- 

 ent. Two pairs of antennae. Res- 

 piration branchial (by means of gills) 

 or cutaneous (directly through the 

 skin). The exoskeleton contains car- 

 bonate and phosphate of lime in addi- 

 tion to chitin. Example, cray-fish. 



Arachnida. Terrestrial. Usually 

 two regions, cephalothorax and abdo- 

 men; though various Acarina have but 

 one and Solpugida have all three 

 head, thorax and abdomen. Cephalo- 

 thorax unsegmented, bearing two pairs 



of oral appendages and four pairs of legs. Abdomen segmented or 

 not, limbless. Respiration tracheal, by means of book-leaf tracheae, 

 tubular tracheae, or both; stigmata almost always abdominal, at most 

 four pairs. Heart abdominal in position. Example, Buthus (Fig. 2). 



Malacopoda. Terrestrial. Vermiform (worm-like), unsegmented 

 externally. One pair of antennae, a pair of jaws and a pair of oral slime 

 papillae. Legs numerous, paired, imperfectly segmented. Respiration 

 by means of tubular tracheae, the stigmata of which are scattered over the 

 surface of the body. Numerous nephridia (excretory) are present and 

 these are arranged segmentally in pairs. Two separate longitudinal 



FIG. 2. A scorpion, Buthus. 

 size. 



Natural 



