LABOEATOEY EXAMINATION OF STOMACHS. 13 



beaker. The beaker is then held under a faucet, so that the rush of 

 water will cause whatever insect remains tciSjJ be present to float to 

 the surface, where they can be decanted oft* with a filter of bolting- 

 cloth. The remaining matter in the beaker (generall}^ seeds and fruit 

 skin) is then collected on another filter, and from the two filters the 

 material is transferred by means of a scalpel to separate smooth blot- 

 ters about o inches square, and is ready for examination. Examina- 

 tions are usually made with a dissecting microscope furnished with 

 an achromatic triplet lens, but occasionally it is necessary to emploj^ 

 the liigher powers of tlie compound microscope. 



The ijrincipal difficulty arises from the fact that birds often mutilate 

 their food before swallowing it, and the gizzard afterwards reduces it 

 to fine fragments. A song sparrow, for exami)le, will seize a grass- 

 hopper, pinch it a dozen times, pull off and eat the head, pull off the 

 legs and wings and then swallow the abdomen, leaving the otlier 

 13arts. In the gizzard, with its powerful muscular walls, the reduc- 

 tion of the insect is more complete, so that 

 usually within two hours only a few bits of 

 grasshopper dust remain. It is with such ma- 

 terial that the examiner has most often to 

 deal. But with practice iiis eye quickly detects 

 amidst tliis dust a squarish, bicolored jaw with 

 a grooved cutting-edge behind which is a 

 grinder (see PL II, fig. 1). If the jaw is lacking, 

 a little search seldom fails to re>^eal a tiny piece 

 that looks like a human ear, but in reality is 

 part of the knee-joint of the grasshopper (see 



PI IT fi<J' ']\ Pig. 7.— Jaw of May-beetle 



' ^' '' (top and side views I. 



The remains of caterpillars found in bird stom- 

 achs usually consist of the discolored broken skin, which has been 

 twisted and rolled into compact little packets by tlie action of the stom- 

 ach. Sometimes nothing is left by which to identify the insect except 

 the concave jaws, the prominent spherical condyles of a\ hich, however, 

 are unmistakable (see PI. II, fig. 8). Butterflies and moths nia}^ be dis- 

 tinguislied by the tiny tooth-scales of the wing (see PI. II, fig. 2) when 

 the naked eye is unable to detect the i:)resence of these insects. Beetles 

 resist digestion more than cateri:)illars and grasshoppers, consequently 

 pieces of tiieir hard shells ma}' be found in the stomachs for some time. 

 Tliese and other fragments serve to distinguish the different kinds. 

 The hard parts of the genital organs of different sj^ecies of May-beetles 

 (see PI. II, fig. 4) are very distinctive in character, and so afford 

 ready means of identification. The blunt, curiously shaped jaws (see 

 fig. 7) are also characteristic. The hinged bod}^ of a click-beetle is 

 provided with a tooth which strikes against half of the hinge and 

 causes the click that is heard as tlie beetle springs into the air (see 

 PI. II, fig. -4). This tooth when met with in a bird's stomach is often 



3507— No. 15—01 2 



