110 
PROCEEDINGS Ofl THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
of butterflies which are very abundant. Owing to their : 
possessing a powerful odour, which is peculiar to the cater- j 
pillar, chrysalis and perfect insect, if a specimen be J 
crushed between the fingers, a yellow fluid oozes out, hav- j 
ing a strong pungent smell, which stains the skin, and on j 
account of which no birds will eat these insects. Many j 
species in this order are imitated, but I will only : 
mention one, Me’hone Psidii, which is mimicked in size, 
colour, and markings by Leptalis Orise, belonging to a 
different order altogether,— viz,, the Pieridce, or the order 
to which our common whites belong. In all cases where 
one species mimics another, the two always inhabit the 
same district, and the mimicking species is always very 
much less abundant than those it imitates. “ In propor¬ 
tion,” says Mr Wallace, “ they are often not one to a 
hundred, and sometimes not one to a thousand.” The 
Duke of Argyll, writing from Cannes under date of j 
9th November, gives another very remarkable instance j 
of mimicry in the case of a moth which settled on | 
the ground in front of where the Duke was sit- j 
ting, and which was then a very conspicuous object, but 
on becoming alarmed, by turning slightly round and giving 
a violent jerk to the wings, it immediately became invisible, j 
as it then imitated the withered and crumpled leaves by j 
which it was surrounded. “ Here, again,” the Duke adds, 
“ was one of those cases of mimicry in which the com- j 
pleteness of the deception necessitated the co-operation of 
the insect’s own will.” (See Nature of 7th December last.) 
The subject we have considered to-day is one of 
great extent, but I venture to think that the cases I 
have brought under your notice will have been sufficient 
to convince you that the forms and the actions of 
insects have not been given to them without serving a 
useful purpose, and that their colours have not been scat¬ 
tered in any careless or haphazard fashioD, but that all 
serve them good purpose, either affording them protection 
from danger, or helping them to procure the necessaries of 
life ; and that they also play a wonderful part in helping 
to keep that harmony of Nature in which all are so much 
interested. 
The lecture was illustrated by several drawings and 
specimens of butterflies and moths. 
On the motion of the Rev. Dr Milroy, seconded by Mr 
J ohn Macgregor, a cordial vote of thanks was awarded 
Mr Ellison for his valuable paper. 
SUMMER SESSION, 188 3. 
The following Excursions were made :— 
24th May. 
1. Banks of the Tay below Errol. 
As in the physiography of Perthshire the River Tay is a 
very important feature, it has been the custom of the 
Perthshire Society of Natural Science to dedicate, if 
possible, one or more of its excursions each year to an 
exploration of some part or other of the banks of the 
river. As is well known, these present very different 
aspects, accordingly as they are near or are remote from 
the mouth of the river, and it can be well imagined that 
the fauna and flora is equally various. Of course, certain 
plants and animals occur throughout the course of the 
Tay, but many are confined to particular parts, and even 
the individuals representing species that are to be found 
from the source to the mouth of the river are not un- 
frequently characterised by local peculiarities arising from 
the differences of soil, climate, &c., that the different parts 
of the banks possess. Eor example, the individuals of a 
common plant which, in the middle parts of the course of 
the river, does not present any special peculiarities, often 
take on in the upper parts of the course a more dwarf habit 
and larger and brighter coloured flowers; while nearer the 
estuary the leaves are sometimes apt to become more 
fleshy. Such variations may seem to be of little moment, 
but, when regarded in the light of the evolution theory, 
they become interesting, and as the Society is always 
desirous of inculcating on its members the importance of 
studying in the field the facts brought before them in the 
lecture-room, excursions to various parts of the river banks 
will always afford material for investigation and discussion. 
The first excursion for the season took place on Thurs¬ 
day, when the banks of the estuary from Longforgan to 
near Port-Allan were visited and examined. Perthshire 
having no seaboard proper it is only in the lower parts of 
the river that any approach to a maritime or marine flora 
and fauna can be found, and that, moreover, rather re¬ 
stricted in its nature. 
The excursion was conducted by Dr Robertson, of 
Errol, who, it will be remembered, read an interesting and 
