Why can't arthropods grow to be very large ?

Arthropods have an open circulatory system. This type of
system is very inefficient and would limit the size to which an organism can attain.
Their heart pumps blood to their body cavity, where the tissues of the body are bathed
in blood. Diffusion occurs and substances are supplied to the cells, in a slow manner.
This system works well with a smaller body plan. However, in a larger, more complex
organism, a more efficient closed circulatory system would be needed to rapidly supply
cells with their needs. Respiration is also less efficient in Arthropods. Tracheal tubes
carry oxygen from opening in the exoskeleton called spiracles directly to cells which
obtain it by diffusion. For aquatic arthropods, there are gills, however, the blood
doesn't have hemoglobin to transport oxygen, as seen in humans, for instance. Therefore,
although their respiratory and circulatory systems are efficient enough to meet their
needs, it also places a limit on the size that Arthropods grow. Arthropods possess a
tough exoskeleton that cannot stretch and thus, this can restrict growth. However, they
shed or molt the exoskeleton several times during their lifespan. All of these factors
affect the size that Arthropods attain.