256 Adjustment to Conditions of Aquatic Life 



mandibular tusks projecting forward from beneath 

 the head. It thrusts forward its approximated blade- 

 Hke fore feet, and with them scrapes the sand 

 aside, making a hole. Then it thrusts its tusks into 

 the bottom of the hole and lifts the earth forward and 

 upward. Then, moving forward into the opening 

 thus begun and repeating these operations, it quickly 

 descends from view. 



Squeezing thro the bottorn is the method of progress 

 most available to soft-bodied animals. Those lacking 

 hard parts such as shovels and tusks with which to dig 

 make progress by pushing a slender front into a narrow 

 opening, and then distending and, by blood pressure 

 enlarging the passageway. The horsefly larva shown in 

 figure 137 on page 230 (discussed on page 22"]) is a good 

 example. The body is somewhat spindle-shaped, taper- 

 ing both ways, and adapted for traveling forward or 

 backward. It is exceedingly changeable in proportions 

 being adjustable in length, breadth and thickness. 

 Indeed, the whole interior is a moving mass of soft 

 organs, any one of which may be seen thro the trans- 

 parent skin, slipping backward or forward inside for a 

 distance of several segments. The body wall is lined 

 with strong muscles inside, and outside it bears rings 

 of stout tubercles, which may be drawn in for passing, 

 or set out rigidly to hold against the walls of the burrow. 

 The extraordinary adjustability of both exterior and 

 interior is the key to its efficiency. When such a larva 

 wishes to push forward in the soil, it distends and sets 

 its tubercles in the rear* to hold against the walls, and 

 drives the pointed head forward full length into the mud ; 

 then it compresses the rear portion, forcing the blood 



*Certain cranefly lan.-£E (such as Pedicia albivitta and liriocera spiiiosa) that 

 live in beds of gravel have one segment near the end of the body expansible to 

 almost balloon-like proportions, forming a veritable pushing-ring in the rear. 



