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1960

Journal of the Lepidopterists' Society

63

A METHOD FOR ESTIMATING THE WING RADIUS IN LEPIDOPTERA

by P. H. H. Gray

In measuring the wing radius of spread Lepidoptera, or their antennae, by means of a ruler, dividers, or callipers, there is a risk of damaging the specimens. C. B. Williams developed a device for measuring specimens through the glass cover of the cabinet drawer; it consists of two plates of glass with identical scales on each, one superimposed on the other in exact register, about 1.5 cm. apart. This appears to be a simple solution for measuring the expanse of the spread wings, or the radii of light-coloured wings, but is difficult for radii of such species as Vanessa cardui L., Nyrnphalis antiopa L., and many others whose wing-bases spring from a mass of dark hairs in which the zero points of the scale are lost.

The author has found it possible to obtain satisfactory measurements of wing radii, and lengths of straight antennas, by placing a translucent ruler on the glass cover, provided that the distance between the plane of the object and the scale marks on the ruler is known. The discrepancy between the observed and the true values, caused by parallax, can be overcome by the application of a divergence-factor to the observed values.

Fig. 1. Measuring chamber.

A measuring chamber, simulating a fraction of a Cornell-type cabinet drawer, was made as follows (see Fig. 1): the top was removed from a rectangular wooden box, of Vi" material, measuring 8^4" by 3l/2", by 2" high inside. An opening 21/^" wide was made in the middle of one side, to admit light. Lengthwise slots were cut in the top edges of the two long sides to

64

Gray: Wing radius

Vol.14: no.l

accommodate two sides of a 3V4" square glass plate (A) ; they were cut as deep as the thickness of the glass. The edges of the plate lying in the slots were bound with gummed ipaper tape (B), with narrow straps of the same to serve as hinges (B, B) on the far (window) side.

A pinning card of compressed paper pulp is shown in dotted outline in the diagram, resting on microscope slides (C), which are held in place on the box floor by paper straps; the slides ra:'se the card so that the points of the pins are not damaged. The distance from the pinning surface to the top of the glass plate should be 4 cm. The author uses two layers of slides to adjust the card to the required height. A card of the shape shown is easy to move horizontally. For observing specimens mounted with the underside upwards, a small piece of plasticine can be used to hold the head of the ipin; it is shown in Fig. 1 as embedded in a hole (P) in the pinning card.

In operation the pinned specimen is placed with the wing to be measured as nearly horizontal as possible. A zero mark on the translucent ruler, a cm./ mm. scale, is placed so that it (say the 5 cm. mark) is coincident with the base of the wing when viewed with one eye, at normal vision height (the author uses a Magni-focuser No. 7) then, without moving the head, the position of the apex of the wing in relation to another mark on the scale is noted. For example, the apex of a forewing with radius measured directly as 27 mm. may coincide with the 25 mm. mark from zero; one of 25 mm. with the 23 mm. mark. If both wings lie in the same plane below the scale only one factor is required to convert the observed into the true values; if they lie in different planes factors applicable at these different planes must be found.

This was done as follows: a duplicate scale, of white celluloid with black markings, was placed below the plate at various distances below the plane of the observer's scale; these distances were provided by different numbers of microscope slides arranged in a pile on the pinning card; the exact distances were determined by means of an identical scale held vertically with the marks in juxtaposition with the marks on the observer's scale. The following results were obtained:

Observer's scale                               Divergence

zero at 50 mm. (a) ;                                factor

100 mm. mark at                                  b — a

basic scale mm. (b)                                     ^

10                                               101.5                                                  1.03

15                                                102.5                                                  1.05

20                                               103                                                      1.06

25                                                104                                                     1.08

30                                               105.5                                                  1.10

Allowance has also to be made for divergence at different horizontal distances from the zero point. The averages of the horizontal divergences, at 20, 30, 40, and 50 mm. distances, at each vertical distance, brought the divergence factors to 1.04, 1.05, 1.07, 1.08, and 1.10.

Distance

below observer's scale, mm.

1960

Journal of the Lepidopterists' Society

65

I

IB 3

10-

V)

TtS g

i^ >P

■s*

rj

VJ

<o

ZO

^X

-tr

.\.

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u

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1-

01

02 O?

os oy 10

04- Of 06 OJ

factors

The nomogram above was constructed from the above 'corrected' factors. This allows for insects fixed on pins between LO and 2.5 cm. below the head; the useful factors thus lie between 1.04 and 1.10 for normally pinned insects.

The following tests were made with butterflies: 1. Wings at 20 mm. below observer's scale; factor 1.07.

2. Wings at various distances below observer's scale; random specimens of C. philodice.

66

Gray: Wing radius

Vol.14: no.l

The average values from 23 specimens, of which the above seven are a part, were as follows; by direct measure 25.98 ± 0.42; by estimation 25.76 mm. A random series of C. eurytheme, 5 males and 4 females, was measured and found to have a mean forewing radius of 25.8 mm; the radii were then estimated after readings through a glass cover on the storage box, as well as through the glass of the small box; the results were, for measurements in the storage box 25.6 mm., and for those in the small box 25.5 mm.

Comparative measurements and estimates have so far been quoted for objects lying between 20 and 40 mm. For comparisons of objects extending less than 20 mm. the antennae of 10 F. cardui were examined by both methods; the mean of the measured lengths was 15.9 mm. and that of the estimated was 16.0 mm.

In order to demonstrate the correlations of values obtained by the two methods the figures for forewing radii and antennal lengths of 9 Colias philo-dice males, reared in Quebec Province in November 1952, from eggs laid by one female, are given below:

Means and their standard deviations'. for wings, Estimated: 24.01 ± 0.279 Measured: 24.26 ± 0.396 for antennae, Estimated: 9.3 ± 0.14 Measured: 9.2 ± 0.14

Correlation coefficients:

for wings, Estimated X Measured, r^= 0.9827 for antennae, Estimated X Measured, r := 0.8875

n t= 7, P < .01 for both sets.

A method of this kind should be useful to students of other orders of insects as well as to lepidopterists.

Reference Williams, C. B. 1943. A safe method of measuring the wings of set butterflies. Proc. Roy. cnt Soc. London 18:3-5.

R. R. 2, Digby, Nova Scotia, CANADA