32 KOOT-KNOT AND ITS CONTROL. 



before the new skin was formed. At first the remnants of the old 

 skin are visible as an empty skin attached to the rounded posterior 

 portion of the nematode (PI. I, fig. 9), but soon the growiih of the 

 latter obUterates the cavity left and all signs of it disappear. The 

 anus, which before this time occupied a median ventral position some 

 distance anterior to the tip of the tail, now becomes terminal, and 

 immediately ventral to it but also occupying a position almost ter- 

 minal on the rounded posterior portion appears the prominent genital 

 opening, a horizontal opening with two rather thick and prominent 

 lips (PI. I, fig. 10). The anterior portion has undergone but little 

 change. Apparently fertiUzation must take place at about tliis 

 time, for soon the external genitalia become so modified that this 

 would become impossible. The lips become smaller, the opening less 

 prominent, and eggs begin to develop. 



Up to the last molt the larvae of both sexes are alike, at least ex- 

 ternally. The WTiter's very numerous observations do not allow 

 him to confirm the statement of Atkinson ^ that the female can be 

 distinguished before tliis period by the lack of a pointed tail, that of 

 the male being pointed. In all the wTiter's observations, as pre- 

 viously described, the larvae are indistinguishable until the last 

 molt. Then the still small but sexually mature female may be seen, 

 without a tail, in the old larval skin which has a tail. 



ADULT FEMALE. 



The mature female rapidly increases in thickness, becoming 

 eventually flask shaped to pear shaped with a length of 400 to 1,300 // 

 and a thickness at the point of greatest diameter of 270 to 500 /i, 

 or even 750 fx (PI. I, fig. 12). The average of many measurements 

 is about 800 /x for the total length, 500 /t at the point of greatest diam- 

 eter, the length of the less enlarged anterior portion being 240 fi 

 and its diameter just before the region of great thickening begins 

 150 //. This not greatly enlarged anterior portion usually extends to 

 a Httle posterior to the bulb. The body then enlarges abruptly, 

 this posterior portion being approximately spherical. 



Up to the last molt the spear of the female retains the dimensions 

 and shape it had in the larva. As is characteristic of all spear- 

 bearing nematodes, the old spear is shed with the cuticle at the time of 

 molting, a new spear being formed in its place. Tliis new spear is 

 usually smaller both in length and thickness than the larval spear, 

 and the knobs at its base are less prominent. It is usually 10 to 12 ;u 

 long as against 12 to 15 // (rarely 10 //), characteristic of the larva, 



» Atkinson, 1889. 

 217 



