How do mammals maintain posture? Joints in mammals have two opposing muscles. A low level of contraction in each muscle balances the contraction of the opposing muscle. Two opposing muscles must work together to hold a joint at rest.
Mammals are large enough that the volume of the limbs accommodates two sets of muscles. In contrast, insect limbs are much smaller and may narrow toward the distal ends. The volume in a tarsus or tibia of very small insects may be insufficient to accommodate opposing muscles. How do insects maintain posture without opposing muscles?
The answer is in the structure of the exoskeleton and regulation of muscle contraction. Joints contain a protein, resilin, that has a high degree of resiliency. Muscles that contract to move an appendage are opposed by the resilin-containing joint that returns the appendage to its resting position in the absence of muscle contraction. Insect muscles do not contract in all or nothing fashion like most muscles in mammals. An insect muscle can be held at any state of partial contraction. When the muscle contracts, it pulls the joint to the desired position. When a muscle relaxes, the resilin pushes the joint back. The degree of muscle contraction is adjusted to obtain the desired posture.