DYTISCUS MARGINALIS. THE GREAT WATER BEETLE. I51 
from it. The largest tubes are those which run along the sides of 
the insect and open into what are known as “spiracles,” or breath- 
ing pores. ‘The spiracles through which the air enters the tracheze 
are generally visible on the abdominal segments of the insect, as a 
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Fig. 24. 
series of pores along the margin of the under surface. In the 
larva each segment is provided with a pair of spiracles, but in the 
perfect insect several in the thoracic region remain closed, and so 
the number is reduced. The spiracles, generally speaking, are 
furnished with some kind of sieve at their entrance by which par- 
ticles of foreign material are kept out, which would soon, if allowed 
to enter, silt up the tubes. A spiracle may be seen in fig. 25. 
The second portion of the definition of an insect was, that it 
must be divided into three parts, viz., the head, the thorax, and 
the abdomen. We may now notice the transformations which the 
insect undergoes before reaching the adult state. First, we find 
the egg, then the larva (the caterpillar or grub), the pupa (or 
chrysalis), and the imago, or perfect insect. The two phases of 
existence which are most readily observed, and which are certainly 
the most interesting, are the larva and the i imago or perfect insect. 
The larva is made up of a number of segments enclosed in a series 
of double half rings, joined at their margins by a membrane. The 
larva casts its skin several times as occasion requires, for he seems 
to do nothing but eat, and as he increases in size he expands his 
coat to the uttermost till it arrives at the breaking strain, when it 
gives way and the larva emerges enveloped in a new skin which 
has been forming under the old one. ‘The whole growth takes place 
while the creature is in a larval state, for the perfect insect does 
