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1952
The Lepidoptensts' News
101
BUTTERFLIES AND HILLTOPS
by James R. Merritt
In a recent note by RAWSON (Lep. News, vol. 5: p. 70) Anthocaris midea Hbn. is reported in New York as frequenting the top of an outcropping of rock some thirty feet higher than the surrounding terrain. A comment by the Editor suggested that the presence of the species might be explained by the presence of its food plant, rock cress. In a subsequent note by Mather (Lep. News, vol. 6: p. 42) A. midea is reported as showing no preference for hilltops in Mississippi. To this material I should like to add some observations of my own and pose some quaeres concerning "hilltopping" generally.
Near Louisville, Kentucky, lies a range of low hills called the Knobs which rise rather abruptly to an elevation of 850 feet, which is 400 feet above the surrounding bluegrass plain. In the spring A. midea is not uncommon along the tops of the ridges. All of the specimens observed on the ridges — say 100 individuals — were males. Lower down on the sides of the hills I have taken two females, but I have never seen a male except right on top. Both males and females have been taken on the bluegrass plain but are rare. On the ridges several males will fly in a kind of pattern or route which is roughly circular and sufficiently predictable that it is possible to station one's self with reasonable assurance that an opportunity to capture a specimen will be afforded. These males fly one to four feet above the ground and rarely climb higher unless disturbed. Not all ridges in the Knobs are favored with a flight of A. midea. For example, at one point three ridges converge and at the point of confluence there is a small flat. In six visits covering three years, I found A. midea on the flat and along the north and west ridges but never on the south ridge.
In these same hills Papilio marcellus Cram, (early spring form) shows a similar preference for the tops of the ridges. This species also flies at a rather constant level about three and a half feet above the ground. Of about twenty specimens captured, all were males. Although I have visited these ridges many times during the summer, I have never observed the summer form of P. marcellus flying in a similar pattern.
To a more limited extent Papilio glaucus L. and Papilio polyxenes asterius Cram, also fly in patterns about these same ridges. P. glaucus does its flying at a considerably higher level, rarely descending below seven or eight feet. P. p. asterius flies only on the more open ridges where the timber has been cleared away.
In Colorado I have observed similar phenomena in the foothills near Boulder. There, however, the species were Euchloe olympia Edw. and Papilio eurymedon Luc. Again only males wrere involved.
In New Mexico above the town of Raton some hills rise to an altitude of 7,500 feet. At the very tops of these hills I have found Papilio polyxenes asterius (?) demonstrating the same behavior. On one hill I frequently captured Papilio philenor L. flying a pattern about the top. In eight years collecting in Raton I never took a P. philenor in any spot other than the top of that one hill. One or two individuals, however, could be found there with great dependability.
102 MERRITT: Butterflies and Hilltops Vol.6, nos.6-8
In California Papilio zelicaon Luc. is commonly found on the tops of hills. Frequently I have found a group of several males flitting about a bare outcropping of rock on some elevation.
My observations in Kentucky, Colorado, New Mexico, and California, although limited, seem to warrant several findings of fact: 1. Some butterflies are more numerous on the tops of hills. 2. Groups of individuals appear to fly in roughly defined patterns about hilltops. 3. These butterflies are mostly males. 4. The butterflies are either Papilionidae or Pieridae.
Assuming the validity of my findings, what is the explanation? Various hypotheses suggest themselves readily.
1. They are attracted by the food plant. If the food plant were the attracting agency, females should be found, whereas, my observations have disclosed only males. Furthermore, it is difficult for me to believe that food plants in the area would keep the butterflies on the tops of the ridges. The food plant supposition also fails to explain why the butterflies fly in patterns.
2. They are blown to the ridges by the wind. This might explain how the butterflies got there, but why do they stay there? Besides, I have known the butterflies to hover about hilltops on windless days.
3. They are subject to a tropism. To explain this behavior by a tropism is a teleological explanation and little better than stating that these butterflies fly around hilltops because they fly around hilltops. The "why" inquiry still remains.
4. They represent surplus male population like butterflies congregating about puddles. Why butterflies go to puddles has the simple explanation that they are thirsty, but I know of nothing to be satisfied by climbing to the top of a hill.
5. They just like hilltops. My friends, the biologists, will insist that I repudiate this suggestion as "anthropomorphic" even though it explains my own presence on the hill.
None of these hypotheses seems tenable to me and I should be interested in the true explanation, or in other theories.
School of Law, University of Louisville, Louisville, Ky., U.S.A.
Editor's note: Mr. MERRITT has not mentioned two important reasons, both suggested by Mr. ARNHOLD'S paper, why the butterflies might be on hilltops: first, if the food-plant is found only there (and he gives no information on foodplants), that is where emergence from the pupa occurs and where fresh adults may be most numerous; second, and correlated with the first, if the foodplants are concentrated there (which is true for Anthocaris and Eucbloe in Connecticut and Missouri), and if, as recent observations indicate, females mate almost immediately on emergence, then males successful in finding mates must patrol the hilltops. Females of Euchloe and Anthocaris begin to fly long after the males, so the lack of females on a hilltop may be solely a result of the date of the collector's visit. The Papilionidae and the Nymphalidae which also abound on high pinnacles seem to me to be there for a very different reason from the Euchloini. What we need most are actual counts of individuals of these various species, counts made on the hilltops, the slope, and the base on a series of dates. Notes of such observations are invited from lepidopterists in any part of the world.
C. L. Remington.