20 NATURE 
THE LIFE-HISTORY OF A WATER- 
BEETLE. 
‘THE life-history of a water-beetle can be outlined 
in a very few words. An egg is laid by the 
mother-beetle: an aquatic larva hatches out which 
feeds and grows, and, during the process of growth, 
moults several times. When full grown it leaves the 
water and burrows into the earth, forming a ‘‘ cell,” 
in which it changes to a pupa. After a time the 
pupal skin is cast off, and the perfect insect makes 
its way out of the cell and resumes its life in the 
water. 
There are, however, all sorts of interesting details 
in the life-history, and these details often differ con- 
siderably in different types. There are differences in 
the egg-laying habits; differences in the method of 
development of the embryo; differences in the way 
the larva gets out of the egg; differences in the way 
it feeds and in the nature of its food, and so on; and 
it is these differences which are of importance to each 
species in enabling it to fit in among other species in 
the life of the community. 
Although there are a number of widely separated 
species of beetles which inhabit the water, there are 
two groups which are usually referred to as ‘‘ water- 
beetles,” and these may be broadly distinguished as 
the swimming carnivorous group—the Hydradephaga 
—and the creeping herbivorous group—the Palpi- 
cornia, or Hydrophilide. The description of this 
second group is not strictly accurate, as the larve 
are, apparently without exception, carnivorous, and 
the perfect insects, although capable of subsisting 
upon a vegetable diet, in at least many cases enjoy 
animal food; and although they are somewhat differ- 
ently constructed from the swimming water-beetles, 
some of them are very fair swimmers. 
I propose to outline the life-history of a type of the 
Hydradephaga, and then to compare with it a type 
of the Palpicornia; and as a type of the former group 
I will describe a species of Dytiscus, D. lapponicus, 
the life-history of which I worked out during last 
summer. 
The male and female differ in general appearance, 
the former having smooth wing-cases, the latter 
having these grooved or fluted. The male has also 
a pad on each of the front legs, while the female 
has quite simple front legs. The slide also shows a 
full-grown larva, and thus gives an idea of the rela- 
tive sizes of these two stages of the species. 
This species is extremely local in. the British Islands, 
only having been found in a few localities in Scot- 
land, and in one in north-west Ireland. It inhabits 
lochs, usually mere lochans, at altitudes of from 
800 ft. upward, and there are certain characteristics 
about its habitat which make it possible generally to 
tell at a glance whether a particular lochan is or is 
not likely to hold the species. 
As a rule the habitat is a bare stony lochan, with 
very little vegetation; it has no stream flowing into 
or out of it, and trout and lapponicus are mutually 
exclusive. There are usually newts and fresh-water 
shrimps (gammarus), but otherwise there is always a 
marked scarcity of animal life. Very few other water- 
beetles are associated with lapponicus, which usually 
is abundant where it occurs. . 
The only place I have found the species in great 
abundance is in a lochan aso ft. above sea-level on the 
island of Eigg. Along its eastern side this lochan 
4s strewn with large stones, and under these the 
heetle is to be found, often as many as four or five 
under one stone. It occurs in other lochans on Eigg, 
1 Discourse delivered at the Royal Institution on Friday, Mey o, by 
F. Falfonr Browne. 4 
NO. 2288, VoL. 92] 
[SEPTEMBER 4, 1913 
and has been found also in Rhum, Skye, Mull, and 
, Arran, but otherwise it is only known from Inverness- 
| shire, ° 
One place in Mull where it used to occur abundantly 
is a peculiar loch, situated in the top of a hill, about 
800 ft. behind Tobermory. The place looks like the 
crater of a volcano, but I believe is not so described 
by geologists. The species has apparently quite dis- 
appeared from this loch; it is probably slowly dis- 
appearing from our islands, being a remnant of the 
fauna which abounded when our climate was much 
colder than it is at present. 
All my specimens came from the one lochan on 
Eigg, and they were placed in large tubs in my 
garden in the north of Ireland. The tubs are filled 
with water, but the bottom is covered by a thick 
layer of soil, and in the soil a few species of water 
plants thrive, chiefly the common water-grass, Glyceria 
aquatica. The tubs are covered with wire-gauze to 
prevent the beetles escaping. 
Now the Dytiscus possesses a small apparatus 
capable of piercing the tissues of the water-plants, 
and each time this borer makes a hole in the water- 
plant one egg is deposited. In my tubs the lapponicus 
chose the water-grass as the receptacle for its eggs. 
In its native home this grass does not grow, the only 
water-plants being a common rush, a species of 
juncus, and the club rush eleocharis, both possessing 
round stems. Now, the grass possesses a round stem 
surrounded by leaves, each leaf consisting of a long 
sheathing base and a free lamina or blade. The 
sheath is keeled, and in every case the mother-beetle 
pierced the leaf-sheath, and always in the line of the 
keel, depositing the egg in the tissues of the sheath, 
and this shows the peculiar instinct possessed by the 
mother in the deposition of her eggs and the extreme 
sensitiveness of the borer or ovipositor. Although I 
examined very carefully the plants in the tubs, only 
twice did I find that the ovipositor had passed right 
through the sheath and dropped the egg between that 
and the stem. 
Lapponicus, unlike our other species of Dytiscus 
has a very definite egg-laying period, commencing in 
March and ending in June. From two of the, British 
species I have had eggs in October, December, and 
February, as well as in the summer months. 
I collected a number of the eggs, dissecting them 
out of the leaf-sheaths, and placed them on wet cotton 
wool in tumblers and watched their development. 
I do not intend to weary you with the details of 
the development of the embryo, but I wish to point out 
that the embryo first appears on a part of one side 
of the mass of yolk—it does not at first occupy the 
whole length of the egg—and it then extends first 
backwards and then forwards, and the sides grow up 
around the yolk until the embryo ultimately encloses 
it. The nerve-chord does not increase in length with 
the embryo, and consequently appears to shorten as 
the embryo extends in the egg. 
The development of the embryo occupies about three 
weeks in June, but temperature affects the length of 
this embryonic period. In the case of another species, 
an egg laid in April matured in three weeks, while 
one laid in winter took six weeks to hatch. 
Towards the end of the embryonic period the 
pressure of the embryo in the shell is very great. T 
accidentally punctured an egg with a needle when 
turning it over, and immediately a portion of the 
embryo bulged through, just as the inner tube of a 
pneumatic tyre tends to bulge through a tear in the 
outer cover. The pressure is also indicated by the 
changed shape of the egg during the final stages. 
During the latter part of the egg-period, there are 
various slight movements of the embryo, but during 
| the last few hours certain very definite movements 
