Some insects also use a molecule called haemocyanin, which does the same job as haemoglobin does in vertebrates (but less efficiently). The insect circulatory system has no veins or arteries. The 'blood' is called haemolymph, and moves around in the space called the haemocoel. The organs sit in the haemocoel and are bathed in the haemolymph. The 'heart' is little more than a single tube which pulses (squeezes).