These are microscopic bugs (200 to 600 µm) belonging to the spider family.

In the home, house dust mites nest in dust, bedding, sheets, carpets, curtains and soft toys, since they like heat and humidity. A mattress can contain almost two million house dust mites while up to 2,000 can be found in a gram of dust.

It is not the house dust mites themselves which cause allergies but the excrement they generate.

Allergies to house dust mites are described as "perennial" since they occur throughout the year. However, the symptoms are more intense at certain times of the year, particularly in early autumn (when we live in more confined spaces). 
Links to house/storage dust fact sheets

Stallergenes Greer UK