Bees have no arteries or veins. A tube-shaped "dorsal vessel" running the length of the body pumps hemolymph (insect blood) forward into the head, where it simply spills out into the open body cavity (hemocoel) and drifts back — an open circulatory system. Small accessory pulsatile organs at the base of wings and legs give extra pushes to move fluid into those extremities.
flowSpeed ∝ heartRate / viscosity
Q_accessory = pump_on ? Q_base : 0
open system: no vessel walls beyond the dorsal tube
- Heart rate — pulse frequency of the dorsal vessel; faster heartbeat pushes hemolymph forward more often.
- Hemolymph viscosity — thicker fluid (e.g. cold weather) slows particle drift through the open cavity.
- Flow particle density — how many hemolymph particles are rendered, for visual clarity vs performance.
- Wing / Leg pump toggles — the accessory pulsatile organs; switching them off shows fluid failing to reach the wings or legs.
Real application: understanding open circulation explains why insecticides that enter the hemolymph spread through a bee's whole body almost instantly, with no vessel walls to contain them.