Local cost minimization in ant transport networks: from small-scale data to large-scale trade-offs.

Transport networks distribute resources and information in many human and biological systems. Their construction requires optimization and balance of conflicting criteria such as robustness against disruptions, transport efficiency and building cost. The colonies of the polydomous Australian meat ant Iridomyrmex purpureus are a striking example of such a decentralized network, consisting of trails that connect spatially separated nests. Here we study the rules that underlie network construction in these ants. We find that a simple model of network growth, which we call the minimum linking model (MLM), is sufficient to explain the growth of real ant colonies. For larger networks, the MLM shows a qualitative similarity with a Euclidean minimum spanning tree, prioritizing cost and efficiency over robustness. We introduce a variant of our model to show that a balance between cost, efficiency and robustness can be also reproduced at larger scales than ant colonies. Remarkably, such a balance is influenced by a parameter reflecting the specific features of the modelled transport system. The extended MLM could thus be a suitable source of inspiration for the construction of cheap and efficient transport networks with non-zero robustness, suggesting possible applications in the design of human-made networks.

[Diverticular disease of the colon, the indications for and results of elective surgical treatment]. / Enfermedad diverticular del colon, indicaciones y resultados del tratamiento quirúrgico electivo.

Precise criteria were prospectively used to indicate surgery in patients with diverticular disease of the colon: a) anatomical deformity of the sigmoid (ADS), including stenosis and fistulae with presence of contrast media extraluminally; b) recurrent acute diverticulitis (RAD); c) prior diverticulitis with perforation (DP) and d) recurrent GI bleeding (GIB). 42 patients were operated on according to these criteria, from 1978 to 1988, 26 males and 16 females, with a mean age of 61 years (range 31 to 93). ADS was the indication in 22 patients (52%), with fistulae present in 9. RAD was the indication in 16 patients (37%), DP in 9 patients (21%) and GIB in 5%. All patients were submitted to either ample sigmoid resection or left hemicolectomy. All patients were followed, with a mean of 43 months for the series. Only 1 patient died after surgery from pulmonary embolism. Late clinical evaluation revealed absence of preoperative symptoms and signs. We feel that surgical treatment of diverticular disease of the colon is warranted with the criteria described, with good immediate and longterm results.