334 



SCIENCE. 



[Vol. I., No. 12. 



they will find their food-supply, and opportunity to 

 develop. In this view- of the matter, the develop- 

 ment of such non-homologous parts for analogous 

 purposes is of great morphological interest. The 

 analogy with the young Meloids will doubtless be 

 found to go still farther, in that the young Bombyliid 



Fig. 3. — a, full-grown larva ; b, pupa-sliell ; c, larva igBuing fi*om 

 pupa of Rliizotragus. (After Haudlirsch.) 



will hibernate and otherwise live for a long time 

 without food, waiting patiently for tlie hatching and 

 growth of its intended victim, which growth may be 

 very rapid among lamellicorns and pectinicorns, as I 

 have shown in the case of Passalus coriuitus [5 Mo. 

 ent. rep., 55), in which full larval development from 

 the egg requires but six weeks. C. V. Eilbt. 



LETTERS TO THE EDITOR. 



[Correspondents are requested to be as brief as possible. The 

 ioriier's name is in alt cases required as proof of good faith.] 



IThe Lake-Superior rocks. 



In Science for Feb. 9, Mr. Selwyn refers to what 

 he regards a ' mistake ' of mine, in quoting him as be- 

 lieving that the trap and sandstone of Lake Superior 

 are of the age of the Huronian. The statement was 

 made on the authority of his report for 1877-78, p. 14 

 A., where, in his general classification, he has placed 

 in the Huronian "the typical or original Huronian 

 of Lake Superior, and the conformably — or uncon- 

 formably, as the case may be — overlying upper cop- 

 per-bearing rocks." I could not, of course, in 1881, 

 state what Mr. Selwyn may believe in 1883, regarding 

 the trap and sandstone of Lake Superior. A fair in- 

 spection of the Tenth annual report of the Minnesota 

 survey, whicli he criticises, would have shown him 

 that that opinion was quoted from him in 1877-78, 

 since his report for that year is given as authority for 

 the statement on the following page. Still, I am 

 very glad to be re-enforced in the views which I have 

 advocated in my reports since 1872, first promulgated 

 by Messrs. Foster and Whitney, by the distinguished 

 authority of the director of the Canadian survey. I 

 concur with him in tlie sweeping affirmation, " that 

 there is, at present, no evidence whatever of their 

 holding any other place in the geological series " than 

 that of the ' Potsdam and primordial Silurian ; ' and 

 I would also add, that there is much incontestible 

 evidence that they can hold no other. 



In Science for March 9, Mr. Irving has misquoted 

 and misrepresented my views in three respects: 1. 



That I have strenuously refused to believe in the un- 

 conformability of the sandstone and trap at Taylor's 

 Falls in the St. Croix valley; 2. That, after ray visit 

 to the valley in 1881, I confess to the unconformity; 

 and, 8. Tlrat I have argued a difference of age be- 

 tween the ' eastern sandstone ' of the south shore of 

 Lake Superior, and that of the St. Croix valley. 



In respect to the first of these, it is only neces- 

 sary to refer to the First report of the Minnesota 

 survey (p. 69, 70), ^vhere the unconformity of the St. 

 Croix sandstone on the ti'ap and sandstones is made 

 a strong point in the argument for separating the 

 two under different names. 



Secondly, I should hardly regard that a ' confes- 

 sion,' in 1881, which is the same that I advocated in 

 1872, and, in the interim, on all suitable occasions. 



Thirdly, as to the difference of age between the 

 sandstones of the St. Croix valley and those of the 

 eastern southern shore of Lalce Superior, jirobably 

 Mr. Irving has misapprehended iny argument in the 

 Tenth report, Minnesota survey. Instead of ranking 

 them of difierent age, I have grouped them as of the 

 same age (p. 134), and call special attention to the 

 fact, that the late investigations of Dr. Eominger, as 

 well as the paleontologioal discriminations of Mr. Bil- 

 lings, go to show their identity. I have, however, a 

 strong inclination to concur with Mr. Irving in the 

 opinion that the ' Animikie group' of Thunder Bay 

 is the equivalent of the original Huronian, and have 

 already expressed reasons for such a supposition 

 (Tenth report, p. 95). Some further examination in 

 the northern part of Minnesota is still necessary to 

 establisli the parallelism. N. H. Winchell. 



Minneapolis, Minn., April 2. 



Venturesome spiders. 



In the summer of 1882, while engaged for the U. S. 

 coast and geodetic survey in the triangulation of New 

 Hampshire, I witnessed an exhibition of tight-rope, 

 or perhaps I ought to say slack-rope, performance, 

 that somewhat surprised meat the time, and which 

 may, perhaps, be of interest to your readers. It was 

 upon the summit of one of our New-Hampshire bills, 

 some 1,400 feet above sea-level, bearing the name of 

 Blue Job. The air was clear, and the sky partially 

 overcast with cumuli clouds, with a very light breeze 

 from the east. After completing a series of meas- 

 urements upon an angle, I stepped for a moment to 

 the western side of my observatory (a small wooden 

 structure, with shutters opening breast-high for obser- 

 vation); and, standing near the north-western corner 

 of the building, I observed, starting out suddenly, and 

 at almost the same instant, three spiders, each spin- 

 ning out his single thread as he went, lying, back 

 downwai'ds, upon nothing but the air, and sailing off 

 at an angle of, perhaps, 10° to 15° above the horizon, 

 as if bound to some other sphere. The rate of motion 

 was not more than a third or half metre per second ; 

 and as the air was very clear, and I soon had the ad- 

 vantage of a bright cloud for a background, I was 

 able to watch the dark specks for a long distance. 

 One of them made a partial failure, if his object was 

 a long voyage, for he came to the gi'ound within ten 

 or fifteen metres; while the other two went on and 

 up as far as the unaided eye could follow them, or pei'- 

 haps I shovxld say one of them did, for at last I was 

 obliged to relinquish one, to be sure of holding the 

 other in view. The distance to which the one was 

 followed could not have been, I think, less than fifty 

 metres. 



The question arises. How did they do it? They 

 went, it is true, in the direction of tlie wind, what 

 there was of it ; but this was so light that I judged at 



