. Elements of Comparative Anatomy. DIGESTIVE CAXAL OF AETIIEOPODA. 277 (Campodea)j but four are present in Lepisma. Tlie function of the urinary canals is notably increased in the Insecta., which have a perfect metamorphosis, during their larval stage, as is shown not only by the great development of these organs (Fig. 139, vm), but also by the quantity of urine which is collected in the hind-gut during their pupal stage. This phjenomenon corresponds exactly to that period in which the most intense plastic activity is going on in the organism, in connection with the development of the perfect
. Elements of Comparative Anatomy. DIGESTIVE CAXAL OF AETIIEOPODA. 277 (Campodea)j but four are present in Lepisma. Tlie function of the urinary canals is notably increased in the Insecta., which have a perfect metamorphosis, during their larval stage, as is shown not only by the great development of these organs (Fig. 139, vm), but also by the quantity of urine which is collected in the hind-gut during their pupal stage. This phjenomenon corresponds exactly to that period in which the most intense plastic activity is going on in the organism, in connection with the development of the perfect body. It is clear that the function of the Malpighian canals of the Insecta is not to be found exclusively in the excretion of urine; but that an older hypothesis, according to which they were regarded as organs for the excre- tion of bile also, is not altogether without justification, is seen from the fact that por- tions of this canal have a different struc- ture, while the secretion in these portions is different also. The brownish-yellow, or light-yellow coloration of the urinary canals is due to the substances deposited in the cells of the canal-wall, and is more intense when secre- â tion is more active. Most of the Diptera (Fig. 142, B vm) and Hemiptera have two urinary canals, which are connected with one another; there are six canals in the Lepidoptera, in many Nocturnal flies, and in several Pseudoneuroptera (Termites); there are four to six in the Coleoptera; the Hymenoptei-a are distinguished by a large number of short urinary canals, and indeed hundreds have been found in them, and in many Orthoptera (Fig. 142, A vni). As a rule they are seldom branched; but we often meet with loop-like connections between their ends. They open at apparently very different points, accord- ing to the length of the hind-gut. They open very far forwards in the Cicada?, Diptera, and Lepidoptera. In the Hymenoptera they open just behind the mid-gut. Where the canals unite to form a com
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