136 
ZOOLOGY: C. T. BRUES 
Huss, rediscussing this materiaP derived a parallax of — 0".063 ± O^'-OSO. 
It should be remarked that on most plates Wilsing measured, not the 
nucleus, but the whole nebula, which has a diameter of about 26". 
Professor Barnard has stated,^ that the central star of the nebula is 
variable "to an extent upwards of three magnitudes; at times it has 
appeared as a bright yellowish star of about the 12th magnitude." 
The star appears to have practically the same magnitude on all my 
plates, varying from equality with comparison star 1 to a trifle brighter 
than comparison star 2. The visual magnitude of comparison star 1 
is given by Barnard as 12.6. 
To derive the absolute parallax of the nebula we must add approxi- 
mately 0".002 to the value given above; the resulting parallax of +0".023 
would place the nebula at a distance of about 140 light-years. As the 
angular diameter of the whole nebula is about 26'', its linear diameter 
would thus be of the order of nineteen times that of the orbit of Neptune. 
These results seem to be extremely promising and the program will 
accordingly be enlarged. 
1 Maanen, A. van, these Proceedings, 1, 1915, (187-189). 
2 Bigourdan, G., Bui. Astr., Paris, 26, 1909, (331). 
3 Maanen, A. van, ML Wilson Contrib., No. Ill, 1916, (1-33). 
4 Wilsing, J., Aslr. Nachr., Kiel, 136, 1894, (349-352). 
sHuss, E., Ibid., 178, 1908, (95-98). 
6 Barnard, E. E., London, Mon. Not. R. Astr. Soc, 68, 1908, (465). 
ADULT HYMENOPTEROUS PARASITES ATTACHED TO THE BODY 
OF THEIR HOST 
By C. T. Brues 
BUSSEY INSTITUTION. HARVARD UNIVERSITY 
Communicated by W. M. Wheeler. December 21.1916 
Entomophagous parasites exhibit many remarkable adaptations 
which enable them more easily to locate the hosts necessary for the 
development of their young. Most of them depend upon the acute- 
ness of their senses and we find consequently that they are usually very 
active and commonly exhibit complicated instincts to aid them in their 
search. Many forms deposit their eggs directly within or upon the 
bodies of their host, seeking either the eggs or the larvae of the host 
for this purpose. When oviposition in larvae occurs, it is necessary 
of course that these be located by the parasites after they have attained 
the requisite size. When the eggs of the parasites are placed within 
the eggs of the host, the latter must be located soon after they have 
