To Recognize Bugs 
Body shape
A typical bug shows an oval, more or less flattened shape (Pentatomidae, Cimicidae for example). Depending on the mode of life there exist numerous variations with a bewildering range of shapes and colours from a nearly rounded body (Plataspidae) to mosquito-like shape (Empicoris).
typical:Pentatoma
spherical:Coptosoma
mosquito-like: Empicoris
Wings
Wings can be fully developed (macropterous), reduced (brachypterous) or totally missing (apterous). The forewings are divided in a tough (sclerotisized) front part and a membranous hind part. That gives rise to the name Heteroptera. Hindwings are membranous and lie folded beneath the forewings when in rest. During flight, fore and hindwings are coupled together.
flying: Carpocoris
apterous:Cimex
brachypterous:Dimorphopterus
Scutellum
A triangular plate, the scutellum, lies between the bases of the forewings. It is often enlarged and in extreme cases it covers the whole abdomen (Scutelleridae, Plataspidae for example). In contrast, the scutellum of beetles is small, often very small.
normal scutellum
extreme scutellum form
typical beetle scutellum
Stinging and sucking
All bugs have a specialised rostrum (beak), which arises from the front of the head and which lies directed backwards under the body when not in use. With the rostrum they can pierce plants and animal prey, inject digestive secretions and suck the plant sap or the liquid compounds of its prey.
stinging rostrum
resting rostrum
mouthparts of a beetle
The stink glands of bugs
Virtually all bug species have stink glands, which produce a defensive secretions which consists of long-chain alcohols and aldehydes. It is an effective repellent against predators, smelling bad for people. It is smeared with the legs on enemies or directly sprayed out of the openings, which lie at each side of the metasternum in the adults and on the back of the abdomen in the nymphs.
Giants and dwarfs
Body length in bugs varies from about 1 mm (or less) in some terrestrial families (Dipsocoridae for example) up to 60 mm (or more) in the rapacious aquatic forms (Nepidae, Belostomatidae).
XX giant water bugs (Belostomatidae) from Japan and Malaysia.