The blood of the Cockroach may be collected for examination by cutting off one of the legs, and wiping the cut end with a cover-slip. It abounds in large corpuscles, each of which consists of a rounded nucleus invested by protoplasm. Amœboid movements may often be observed, and dividing corpuscles are occasionally seen. Crystals may be obtained by evaporating a drop of the blood without pressure; they form radiating clusters of pointed needles. The fresh-drawn blood is slightly alkaline; it is colourless in the Cockroach, but milky, greenish, or reddish in some other Insects. The quantity varies greatly, according to the nutrition of the individual: after a few days’ starvation, nearly all the blood is absorbed. Larvæ contain much more blood, in proportion to their weight, than other Insects. _Respiratory Organs of Insects._ The respiratory organs of Insects consist of ramified tracheal tubes, which communicate with the external air by stigmata or spiracles. Of these spiracles the Cockroach has ten pairs--eight in the abdomen and two in the thorax. The first thoracic spiracle lies in front of the mesothorax, beneath the edge of the tergum; the second is similarly placed in front of the metathorax. The eight abdominal spiracles belong to the first eight somites; each lies in the fore part of its segment, and hence, apparently, in the interspace between two terga and two sterna. The first abdominal spiracle is distinctly dorsal in position. The disposition of the spiracles observed in the Cockroach is common in Insects, and, of all the recorded arrangements, this approaches nearest to the plan of the primitive respiratory system of Tracheata, in which there may be supposed to be as many spiracles as somites.[147] The head never carries spiracles except in _Smynthurus_, one of the Collembola (Lubbock). Many larvæ possess only the first of the three possible thoracic spiracles; in perfect Insects this is rarely or never met with (_Pulicidæ?_), but either the second, or both the second and third, are commonly developed. Of the abdominal somites, only the first eight ever bear spiracles, and these may be reduced in burrowing or aquatic larvæ to one pair (the eighth), while all disappear in the aquatic larva of _Ephemera_. [147] The oldest Tracheate actually known to bear spiracles is the Silurian Scorpion of Gothland and Scotland (Scudder, in Zittel’s Palæontologie, p. 738). We need not say that this is very far removed from the primitive Tracheate which morphological theory requires. The existing _Peripatus_ makes a nearer approach to the ideal ancestor of all Tracheates, if we suppose that all Tracheates had a common ancestor of any kind, which is not as yet beyond doubt.