What insect is this?
This is a terrestrial isopod crustacean called a woodlouse (colloquially referred to as pill bugs, potato bugs, roly-pollies, sow bugs, etc.).  



There are over 5000 species in the world with at least 64 in Russia (see Kuznetsova &amp; Gongalsky 2012). 

Habitat: 

Woodlice are typically found in damp, dark places -- in the soil, under rocks/logs/debris, etc.


  Living in a terrestrial environment, woodlice breathe through trachea-like lungs in their paddle-shaped hind legs (pleopods), called pleopodal lungs. Woodlice need moisture because they rapidly lose water by excretion and through their cuticle, and so are usually found in damp, dark places


Description:

As with all Arthropoda, the woodlouse is a segmented animal with a rigid exoskeleton and jointed limbs.


However, this is not an insect! Insects have 3 pairs of legs while isopods have 7 pairs of legs. 


The extended uropods on your specimen as well as the pereon-pleon junction, number of antenna flagella, etc. all are important for IDing to species. (E.g., see here).

Note: although many people associate these arthropods with the ability to roll up into a perfect ball, only woodlice in the genus Armadillidium and in the family Armadillidae can roll up into an almost perfect sphere as a defensive mechanism.

Diet:

Woodlice are generally detritivores, meaning they eat decaying organic matter.  



Citations:

Kuznetsova, Daria M., and Konstantin B. Gongalsky. "Cartographic analysis of woodlice fauna of the former USSR." ZooKeys 176 (2012): 1.