HOW INSECTS WALK ON SMOOTH SURFACES 111 



" A similar adaptation of the position of the legs, which is entirely dependent 

 on the choice of the insect, may also be observed there, if one compels insects 

 which are not provided with corresponding adhesive lobes to run away over 

 crooked surfaces. Fig. 123 shows the footprints of a Blaps when running upon 

 a horizontal plane. Fig. 127, on the contrary, shows the tracks of the legs when 

 going diagonally over a gradually inclined surface. Here, also, the insect holds 

 on with his fore and middle legs (*i, r 2 ) stretched upward, whereby also the 

 impressions on both sides come to lie farther apart than in the normal mode of 

 walking. 



" It will not surprise the reader who is familiar with the gait of crabs, to hear 

 that many insects also understand the laudable art of going backward, wherein 

 the hind legs simply change places with the fore legs. 



" The jumping motion of insects may be best studied in grasshoppers. When 

 these insects are preparing for a jump, they stretch out the upper thigh hori- 

 zontally, clap the tibiae together, and also retract the foot-segment. After a 

 slight pause for rest, during which they are getting ready for the jump, they 

 then jerk the tibiae suddenly backward and against the ground with all their 

 strength by means of the extensor muscles." 



The correctness of Graber's views has been confirmed by Marey by 

 instantaneous photographs (Figs. 128, 129). 



Locomotion on smooth surfaces. How flies and other insects are 

 able to walk up, or run with the body inverted, on hard surfaces has 

 been lately discovered by Dewitz, Dahl, and others. All authors 

 are agreed that this power is due to the presence of the specialized 

 empodium of each tarsus. 



Dewitz confirmed the opinion of Blackwell, that a glutinous liquid 

 is exuded from the apices of the tenent hairs which fringe the em- 

 podium. By fastening insects feet uppermost on the under side of 

 a covering glass which projects from a glass slide, the hairs which 

 clothe the empodia of the foot of a fly (Musca erythrocephala) may 

 be seen to be tipped with drops of transparent liquid. On the leg 

 being drawn back from the glass, a transparent thread is drawn out, 

 and drops are found to be left on the glass. In cases where these 

 hairs are wanting, as in the Hemiptera, the adhesive fluid exudes 

 directly from pores in the foot. In the beetles (Telephorus Caspar) 

 and other insects the tenent hairs on the foot end in sharp points, 

 below which are placed the openings of the canals. The glands, 

 Dewitz states, are chiefly flask-shaped and unicellular, situated in 

 the hypodermis of the chitinous coat ; each gland opening into one 

 of the hairs (Fig. 108) ; they are each invested by a structureless 

 tunica propria, and contain granular protoplasm, a nucleus placed at 

 the inner side, and a vesicle, prolonged into a tube which, traversing 

 the neck of the gland, is attached to the root of the hair ; the 

 vesicle receiving the secretion. Each gland is connected with a fine 

 nerve-twig, and secretion is probably voluntary. Among the tenent 



