Take a close look at sycamore Acer pseudoplatanus buds at this time of year, just as they are beginning to swell, and you'll probably find some of these tiny insects huddled in groups around the edges of the bud scales. They're sycamore aphids Drepanosiphum platanoidis and they will have recently hatched from minute eggs that have survived the winter on the surface of the bark, just below the bud. It's often claimed that hard winters tend to reduce pest numbers, but that doesn't seem to be the case with this insect - buds on sycamores that I looked at today had a large pupulation of these aphids.
As soon as the tender new leaves begin to burst out of the bud scales the aphids will crawl onto them and begin to feed and breed - very rapidly. At this stage of their life cycle the aphids clone themselves, giving birth to live young that already have the next genetation developing inside them when they are born.
This is what the aphids look like at the moment, magnified about forty times. They cluster together with their antennae touching those of neigbouring insects, so any threats are transmitted through the colony very rapidly.
And this is what they do if they are threatened- all stick their tails up in the air. I'm guessing that they must be emitting something that might repel insect predators....
Once the sycamore leaf expands and begins to photosynthesise a winged generation of aphids develops. Later in the season these can migrate from tree to tree. Notice how most of them are lined up along the leaf veins. Their stylets, like hypodermic syringes, are inserted in the phloem inside the veins that transports the sugars, made by photosynthesis, to other parts of the plant. The sugary solution is rich in energy but very poor in other essential nutrients that the insects need for growth and reproduction, so they need to consume vast quantities of it, most of which is emitted from the tail end of the insect as sticky honeydew - and if you park your car under a sycamore on a hot summer's day when it's raining honeydew it will have the tactile qualities of a toffee when you return.