200 INTRODUCTION TO ZOOIOGY chap. 



(3) Ixodidae. The Common Ticks. 



(Flat or globular short-bodied mites vrttt eight legs, 

 with tracheae, and with characteristic "false-head" 

 and mouth-parts.) 

 Ixodes, the Common Sheep Tick. 



(4) Hydrachnidae. Fresh-water Mites. 



(The four pairs of legs are long and hairy and used as 

 swimming organs ; the larva has only six legs.) 

 Hydrachna, Common Red Water Mite. 



Practical Work on Arachnida 



1. In September, hunt for Common Cross Spiders in the garden. 

 Watch the formation of the web and the snaring of flies in it. 

 Whenever a web is found, look for the spider belonging to it, 

 following the control thread from the centre of the web up to the 

 spider's hiding-place. In October and November, search for the 

 cocoons of eggs, and keep them until the eggs hatch. 



2. Bring one spider indoors, enclosing it for a short while in 

 a small glass tube. (The cork should be pierced to admit air.) 

 Examine with a lens, make out all the main facts of external 

 structure. Sketch the spider in different positions. 



3. Suspend the spider from the end of a twig and examine the 

 threads which issue from the spinnerets. Let it fall some distance 

 from the twig, on its thread, and then touch it and observe the way 

 in which it climbs the thread, roUing it up into a ball as it goes. 



Fix the twig upright in a dish of water ; the spider will run 

 down, but finding it cannot escape, it will probably mount the 

 twig again and give out a silken thread, which, if the stick is in a 

 draughty place, will grow longer and longer, and will finally catch 

 on to something. The spider then quickly pulls the thread tight, 

 fixes her end, and then escapes over the " suspension bridge " thus 

 formed. If the stick is set up in a very sheltered spot (covering 

 it with a bell-jar would sufiice) it will be found that the spider is 

 incapable of emitting this thread. 



4. In the spring, look for the clusters of newly hatched spiders, 

 note how they differ in colouring from the adult spiders ; observe 

 their habits. 



5. With a dipping-net secure a few water spiders from a pond 

 or river, and bring them home ; they will thrive in a tank, if fed 

 with an occasional dead fly. The tank must be covered so that 

 they cannot escape. 



The spiders will readily construct their thimble-shaped " nest," 



