320 ^"^ ANIMAL KINGDOM 



pull oft alter stinging such brittle-skinned enemies, so that the guards 

 live to sting again. They also fly out to stnig large animals that approach 

 too closely. 



After three weeks of adult life the wax glands cease to function and 

 the bee becomes a forager. For the rest of her life her primary function 

 is to collect nectar and pollen. On the average workers will live four or 

 five weeks after reaching this stage. 



1 he queen bee has functional ovaries and uses the reproductive 

 apparatus both for oviposition and stinging. Her iegs lack the pollen- 

 collecting apparatus. Other characteristics of the queen and the drones 

 will be discussed in the next chapter where insect societies are con- 

 sidered. 



1 46. The Subphylum Arachnomorpha 



Arachnomorphs have a long and varied evolutionary history. They 

 appeared first in the ocean, then in fresh water, and finally on land. Of 

 the five or more classes only three will be mentioned here. The Xipho- 

 sura (king crabs) are marine, the Eurypterida are believed to have lived 

 in fresh water, and the Arachnida (scorpions, spiders, etc.) are terrestrial. 



Xiphosura. The class Xiphosura was common, although never 

 abundant, during the Paleozoic era. It survives today as a single genus, 

 Limulus, shown in Fig. 16.2. The only American species is L. polyphemus 

 found on the east coast. The superficial resemblance between king crabs 

 and trilobites is striking. The body is flattened with anterior segments 

 fused dorsally to form a shield. In trilobites this prosoma bore dorsally 

 a pair of compound eyes and ventrally one pair of antennae and four to 

 six pairs of legs. The king crabs are generally similar but lack antennae. 

 The prosomal legs of king crabs lack exopodites, which were the gills 

 of trilobites. The remaining body segments of trilobites were free and 

 each bore limbs like those of the prosoma. In king crabs the remaining 

 segments are fused into an opisthosoma and have much modified ap- 

 pendages. 



The anterior appendages of king crabs are the chelicerae (segment 

 3) hanging in front of the mouth in the typical arachnomorph position. 

 The next four pairs of walking legs are also chelate. The last legs end 

 in several stout spines and are used for pushing in sand. On the opistho- 

 soma the limbs are biramous and fused medially to form flat plates. The 

 first plate is an operculum which overlaps and protects the others. Each 

 of the remaining five plates is delicate and bears a pair of book gills 

 formed of many thin lamellae. The telson remains as a free terminal 

 segment, projecting as a long movable spine. 



Eurypterida. Eurypterids were abundant in Paleozoic times, and 

 included a few species as much as nine feet long. They had a prosoma 

 with dorsal compound eyes and six pairs of ventral appendages (Fig. 

 16.29). The first appendages were chelicerae, the next four pairs were 

 walking legs, and the sixth were large paddles for swimming. 



The remaining segments of eurypterids were unfused and divisible 

 into two regions, a middle mesosoma and a posterior metasoma. The 



