Strangely invisible

Charles John Cornish, The naturalist on the Thames, Londres, Seeley and Co, 1902.

Everything will be quiet and motionless at first, for water beasts are very suspicious of movement above them, and all sham dead, or lie quite still, and are strangely
invisible. On the other hand, they have none of the power of remaining
motionless for half-an-hour like land animals. Soon what look like sticks,
but are caddis larva, begin to creep on the bottom. Then more brown
objects, larvae of dragon-flies and water-beetles, detach themselves from
the stems of the plants and cruise up and down seeking what they may
devour. Other creatures feeding and swimming among or beneath the plants
crawl out on to the upper surface, and the water-beetles come up to
breathe, or to play upon the surface. One of the largest of these is a
very fine black beetle, a vegetable-feeding creature. It is most
interesting to see two of them–they generally live in pairs–browsing on
one of the fern-like plants of the Thames. This plant has leaves like fern
blades, each having in turn its own small spikelets. The big beetles work
along the leaf like a cow in a cabbage yard, biting off, chewing, and
swallowing each in succession, and leaving the stem perfectly bare.
Sometimes it looks as if the two beetles were eating for a match, like the
beef-eating contests held in country public-houses, in which the winner
once boasted that he won easily « afore he came to vinegar. »