14 PRINCIPAL HOUSEHOLD INSECTS. 



lighter than water. It remains motionless at the surface, and when 

 disturbed does not sink without eflbrt, as does the larva, but is only 

 able to descend by a violent muscular action. It wriggles and swims 

 as actively as does the larva, and soon reaches the bottom of the jar 

 or breeding place. As soon as it ceases to exert itself, however, it 

 floats gradually ui:> to the surface of the water again. The fact, how- 

 ever, that the larva, after it is once below the surface of the water, sinks 

 rather than rises, accounts for the death of many individuals. If they 

 become sick or weak, or for any reason are unable to exert sufScient 

 muscular force to wriggle to the surface at frequent intervals, they will 

 actually drown, and the writer has seen many of them die in this way. 

 It seems almost like a contradiction in terms to speak of an aquatic 

 insect drowning, but this is a frequent cause of mortality among wrig- 

 glers. This fact also explains the efiQcac}^ of the remedial treatment 

 which causes the surface of the water to become covered with a film of 

 oil of any kind. Aside from the actual insecticide effect of the oil, the 

 larvfB drown from not being able to reach the air. The structure of the 

 pupa differs in no material respect from that of corresponding stages 

 of European species, as so admirably figured and described by the older 

 writers, notably Reaumur and Swammerdam,^ and needs no description 

 in view of the care with which the figures accompanying this article 

 have been drawn. The air tubes no longer open at the anal end of the 

 body, but through two trumpet-shaped sclerites on the thorax, from 

 which it results that the pupa remains upright at the surface, instead 

 of with the head downward. There is a very apparent object in this 

 reversal of the i^osition of the body, since the adult insect issues from 

 the thorax and needs the floating skin to support itself while its wings 

 are expanding. 



In general, the adult insects issue from the pupte that are two days 

 old. This gives what is i)robably the minimum generation for this 

 species as ten days, namely, sixteen to twenty-four hours for the egg, 

 seven days for the lar^ a, and two days for the i)upa. The individuals 

 emerging on the first day were invariably males. On the second day 

 the great majority were males, but there were also a few females. The 

 preponderance of males continued to hold for three days; later the 

 females were in the majority. In confinement the males died quickly; 

 several lived for four days, but none for more than that period. The 

 females, however, lived for a much longer time. Some were kept alive 

 without food, in a confined space of not more than 4 inches deep by 6 

 across, for three weeks. But one egg mass was deposited in confine- 

 ment. This was deposited on the morning of June 30 by a female which 

 issued from the jmpa June 27. IS'o further observations were made 

 upon the time elapsing between the emergence of the female and the 

 laying of the eggs, but in no case, probably, does it exceed a few days. 



^ Even Bonanni, in 1691, gave very fair figures of the larva and pupa of a European 

 species. Micrographia Curiosa, Rome, MDCXCI, Pars. II, Tab. I. 



